SUMMARY OF THE EXCHANGE OFFER

This summary highlights the material information in this prospectus/offer to exchange. To more fully understand the Exchange Offer to holders of S1 Shares, and for a more complete description of the terms of the Transaction Agreement, the Exchange Offer and the Second-Step Merger, you should read carefully this entire document, including the exhibits, schedules and documents incorporated by reference herein, and the other documents referred to herein. For information on how to obtain the documents that are on file with the SEC, please see the section of this prospectus/offer to exchange titled “Where You Can Find More Information.”

The Companies

(See page 30)

ACI

ACI is a Delaware corporation with its principal executive offices located at 120 Broadway, Suite 3350, New York, New York 10271. The telephone number of ACI is (646) 348-6700. ACI develops, markets, installs and supports a broad line of software products and services primarily focused on facilitating electronic payments. In addition to its own products, ACI distributes, or acts as a sales agent for, software developed by third parties. These products and services are used principally by financial institutions, retailers and electronic payment processors, both in domestic and international markets. Most of ACI’s products are sold and supported through distribution networks covering three geographic regions — the Americas, Europe/Middle East/Africa and Asia/Pacific. As of June 30, 2011, ACI had total stockholders’ equity of approximately $280 million and total assets of approximately $614 million. ACI Shares are listed on the NASDAQ Global Select Market under the ticker symbol “ACIW” and, as of October 12, 2011, the last practicable date prior to the date of this prospectus/offer to exchange, ACI had an equity capital market capitalization of approximately $963.9 million. As of December 31, 2010, ACI had a total of approximately 2,134 employees, of whom 1,124 were in the Americas reportable segment, 591 were in the Europe/Middle East/Africa reportable segment and 419 were in the Asia/Pacific reportable segment.

As of the date of this prospectus/offer to exchange with the SEC, ACI was the beneficial owner of 1,107,000 S1 Shares, or 2.0% of the amount outstanding.

Offeror

Offeror, a Delaware limited liability company, is a wholly owned subsidiary of ACI. Offeror is newly formed, and was organized for the purpose of making the Exchange Offer and consummating the Second-Step Merger. Offeror has engaged in no business activities to date and it has no material assets or liabilities of any kind, other than those incident to its formation and those incurred in connection with the Exchange Offer and the Second-Step Merger.

S1

S1 is a leading global provider of payments and financial services software solutions. S1 offers payments solutions for ATM and retail point-of-sale driving, card management and merchant acquiring, as well as financial services solutions for consumer, small business and corporate online banking, trade finance, mobile banking, voice banking, branch and call center banking. S1 sells its solutions primarily to banks, credit unions, retailers and transaction processors and also provides software, custom software development, hosting and other services to State Farm Mutual Automobile Insurance Company, a relationship that will conclude by the end of 2011. Founded in 1996, S1 started the world’s first Internet bank, Security First Network Bank. In 1998, S1 sold the banking operations and focused on software development, implementation and support services. For several years, S1’s core business was primarily providing Internet banking and insurance applications. Then, through a series of strategic acquisitions and product development initiatives, S1 expanded its solution set to include applications that deliver financial services across multiple channels and provide payments and card management functionality.

S1 Shares are listed on the NASDAQ under the ticker symbol “SONE.” S1’s principal executive offices are located at 705 Westech Drive, Norcross, Georgia 30092 and its telephone number is (404) 923-3500.

The Exchange Offer

(See page 55)

Offeror is offering, upon the terms and subject to the conditions set forth in this prospectus/offer to exchange and in the accompanying letter of election and transmittal, to exchange for each issued and outstanding share of common stock of S1, validly tendered pursuant to the Exchange Offer and not properly withdrawn one of the following:

subject to the proration procedures described in this prospectus/offer to exchange and the related letter of election and transmittal. The blended value of the Cash-Stock Consideration as of the close of trading on October 12, 2011, assuming full proration, was $9.68 per S1 Share.

The equity capital markets have been highly volatile and market prices for ACI Shares have fluctuated and will fluctuate, and could be higher or lower than the price of ACI Shares at or after the Expiration Time. Accordingly, S1 stockholders are urged to obtain current trading price information for ACI Shares prior to deciding whether to tender shares pursuant to the Exchange Offer, whether to exercise withdrawal rights as provided herein and, with respect to the election, whether to receive the Cash Consideration or the Stock Consideration or some combination thereof.

S1 stockholders electing either the Cash Consideration or the Stock Consideration will be subject to proration so that 66.2% of S1 Shares will be exchanged for the Cash Consideration and 33.8% of S1 Shares will be exchanged for the Stock Consideration in the Exchange Offer. S1 stockholders who do not participate in the Exchange Offer and whose shares are acquired in the Second-Step Merger will receive the Proration Amount of Cash and Stock Consideration. The elections of other S1 stockholders will affect whether a tendering S1 stockholder electing the Cash Consideration or the Stock Consideration receives solely the type of consideration elected or if a portion of such S1 stockholder’s tendered S1 Shares is exchanged for another form of consideration. S1 stockholders who otherwise would be entitled to receive a fractional ACI Share will instead receive cash in lieu of any fractional ACI Share such holder may have otherwise been entitled to receive based on ten-day volume weighted average trading prices. For a complete discussion of the proration procedure and the treatment of fractional ACI Shares, please see the sections of this prospectus/offer to exchange titled “The Exchange Offer — Elections and Proration” and “The Exchange Offer — Cash In Lieu of Fractional ACI Shares.”

The Second-Step Merger

(See page 44)

The Exchange Offer is being made pursuant to the Transaction Agreement. Pursuant to the Transaction Agreement, after the Exchange Offer is completed, subject to the approval of the S1 stockholders if required by applicable law, Offeror will merge with and into S1.

The Transaction Agreement provides that at the effective time of the Second-Step Merger (the “Effective Time”), the separate corporate existence of Offeror will cease and S1 will continue as the surviving corporation in the Second-Step Merger. The directors of Offeror immediately prior to the Effective Time will be the initial directors of the surviving corporation, and the officers of S1 immediately prior to the Effective Time will be the initial officers of the surviving corporation. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Second-Step Merger; Effect on Capital Stock.”

Recommendation of the S1 Board

(See page 42)

The S1 Board has unanimously (1) determined that the transactions contemplated by the Transaction Agreement are fair to, and in the best interests of, S1 and the S1 stockholders; (2) approved the transactions contemplated by the Transaction Agreement; and (3) determined to recommend that the S1 stockholders accept the Exchange Offer and tender their S1 Shares to Offeror pursuant to the Exchange Offer. The S1 Board unanimously recommends that S1 stockholders accept the Exchange Offer by tendering their S1 Shares into the Exchange Offer. Information about the recommendation of the S1 Board is more fully described in Amendment No. 2 to the S1’s Solicitation/Recommendation Statement on Schedule 14D-9, which is being mailed to S1’s stockholders together with this prospectus/offer to exchange and is incorporated herein by reference. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Recommendation of the S1 Board.”

Reasons for the Exchange Offer and the Second-Step Merger

(See page 39)

ACI believes that the combination of ACI’s and S1’s businesses will create significant value for both ACI’s and S1’s current stockholders. We believe the combination of ACI and S1 is a compelling combination with a number of strategic benefits, including the following:

Value:

At $9.68 per S1 Share, the blended value of the Cash-Stock Consideration as of October 12, 2011, assuming full proration, the Exchange Offer represents (1) a 35.8% premium to the closing sales price of S1 Shares on July 25, 2011, the last trading day prior to the public announcement of Original ACI Merger Proposal, (2) a 34.3% premium to the volume weighted average closing price of S1 Shares over the previous 90 days prior to the announcement of the Original ACI Merger Proposal, and (3) a 24.9% premium to the 52-week high of S1 Shares for the 52-Week Period.

S1 stockholders who elect the Cash-Stock Consideration contemplated by the Exchange Offer will be subject to proration. The elections of other S1 stockholders will affect whether S1 stockholders receive solely the type of consideration they elect or whether a portion of the consideration S1 stockholders elect is exchanged for another form of consideration as a result of the pro ration procedures contemplated by the Exchange Offer. Since the value of ACI Shares fluctuates, the per S1 Share Stock Consideration necessarily could have a value that is different than the per S1 Share Cash Consideration. As a consequence, in the Exchange Offer S1 stockholders could receive a combination of Cash-Stock Consideration with a value that is different from the value of such consideration on the date of this prospectus/offer to exchange, the Expiration Time and the date of the consummation of the Exchange Offer.

Solely for purposes of illustration, the following table indicates the value of the Cash Consideration, the Stock Consideration and the blended value of the Cash-Stock Consideration based on different assumed prices for ACI Shares.

The equity capital markets have been highly volatile and market prices for ACI Shares and S1 Shares have fluctuated and can be expected to continue to fluctuate. S1 stockholders are urged to obtain current trading price information prior to deciding how to vote. The premium represented by the Exchange Offer may be larger or smaller depending on market prices on any given date and will fluctuate between the date of this prospectus/offer to purchase, the Expiration Time and the date of the consummation of the Exchange Offer.

Strategic Rationale:

The Exchange Offer provides immediate cash value to S1 stockholders, as well as the opportunity to participate in the value creation in the Exchange Offer through the receipt of ACI Shares. ACI believes that the complementary nature of ACI and S1 creates a compelling opportunity to establish a full-service global leader of financial and payments software with significant scale and financial strength, including as follows:

The following metrics provide relevant information with respect to ACI’s recent financial performance, as of July 26, 2011, the date of the Original ACI Merger Proposal:

This prospectus/offer to exchange includes summary selected unaudited pro forma combined financial information that is intended to provide S1 stockholders with information relating to ACI’s financial results assuming that ACI and S1 had already been combined.

Conditions of the Exchange Offer

(See page 69)

The Exchange Offer is conditioned upon, among other things, the following:

Ownership of ACI After the Exchange Offer

(See page 63)

Based on ACI’s and S1’s respective capitalizations as of October 12, 2011 and the exchange ratio of 0.3148, ACI estimates that if all S1 Shares are exchanged pursuant to the Exchange Offer and/or the Second-Step Merger, former S1 stockholders would own, in the aggregate, approximately 14.4% of the aggregate ACI Shares on a fully diluted basis. For a detailed discussion of the assumptions on which this estimate is based, please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Ownership of ACI After the Exchange Offer.”

Comparative Market Price and Dividend Information

(See page 23)

ACI Shares are listed on the NASDAQ Global Select Market under the ticker symbol “ACIW.” S1 Shares are listed on the NASDAQ under the ticker symbol “SONE.”

Based on the $28.77 closing trading price per ACI Share on October 12, 2011, the last trading day prior to the date of this prospectus/offer to exchange, the relative value of the Cash-Stock Consideration reflected by this Exchange Offer consisted of $6.62 in cash and $3.06 in ACI Shares per S1 Share as of such date, or an aggregate blended value of $9.68 per S1 Share as of such date, assuming full proration.

The equity capital markets have been highly volatile and market prices for ACI Shares have fluctuated and will fluctuate prior to the Expiration Time, and could be higher or lower than the ACI Share price at or after the Expiration Time. Accordingly, S1 stockholders are urged to obtain current trading price information for ACI Shares prior to deciding whether to tender shares pursuant to the Exchange Offer, whether to exercise withdrawal rights as provided herein and, with respect to the election, whether to receive the Cash Consideration or the Stock Consideration or some combination thereof.

Solely for purposes of illustration, the following table indicates the value of the Cash Consideration, the Stock Consideration and the blended value of the Cash-Stock Consideration based on different assumed prices for ACI Shares.

The prices of ACI Shares used in the above table, and the assumptions regarding the mix of cash and/or stock a hypothetical S1 stockholder would receive, are for purposes of illustration only. The value of the Stock Consideration will change as the price of ACI Shares fluctuates during the Exchange Offer period and thereafter, and may therefore be higher or lower than the prices set forth in the examples above at the expiration of the Exchange Offer and at the time you receive the ACI Shares. S1’s stockholders are encouraged to obtain current market quotations for the ACI Shares and the S1 Shares prior to making any decision with respect to the Exchange Offer. Please see the section of this prospectus/offer to exchange titled “Risk Factors.”

Interest of Executive Officers and Directors of ACI in the Exchange Offer

(See page 79)

Except as set forth in this prospectus/offer to exchange, neither we nor, after due inquiry and to the best of our knowledge and belief, any of our directors, executive officers or other affiliates has any contract, arrangement, understanding or relationship with any other person with respect to any securities of S1, including, but not limited to, any contract, arrangement, understanding or relationship concerning the transfer or the voting of any securities, joint ventures, loan or option arrangements, puts or calls, guaranties of loans, guaranties against loss or the giving or withholding of proxies.

ACI does not believe that the Exchange Offer and the Second-Step Merger will result in a change in control under any of ACI’s stock option plans or any employment agreement between ACI and any of its employees. As a result, no options or other equity grants held by such persons will vest as a result of the Exchange Offer and the Second-Step Merger. Please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Certain Relationships With S1 and Interests of ACI in the Exchange Offer.”

Interest of Executive Officers and Directors of S1 in the Exchange Offer

(See page 80)

In considering the recommendation of the S1 Board regarding the Exchange Offer and the Second-Step Merger, S1 stockholders should be aware that certain directors and officers of S1 may be deemed to have interests in the Exchange Offer and the Second-Step Merger that are different from or in addition to the interests of other S1 stockholders. S1 has informed us that the S1 Board was aware of these interests and considered them, among other matters, in approving the Transaction Agreement, the Exchange Offer and the Second-Step Merger and recommending that S1 stockholders accept the Exchange Offer by tendering their S1 Shares into the Exchange Offer and, if required by applicable law, approve the Second-Step Merger.

As a result of these interests, S1 directors and officers may have reasons for tendering their S1 Shares and, if necessary, voting to approve the Second-Step Merger that are not the same as your interests. S1 stockholders should consider whether these interests may have influenced these directors and officers to support or recommend the Exchange Offer and the Second-Step Merger.

Information on the interests of executive officers and directors of S1 in the Exchange Offer and the Second-Step Merger is more fully described in Amendment No 2 to S1’s Solicitation/Recommendation Statement on Schedule 14D-9, which is being mailed to S1 stockholders together with this prospectus/offer to exchange and is incorporated herein by reference. Please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Interest of Executive Officers and Directors of S1 in the Exchange Offer.”

Source and Amount of Funds; Financing

(See page 70)

The Exchange Offer consideration will consist of ACI Shares and cash (including, cash paid in lieu of any fractional ACI Shares to which any S1 stockholder may be entitled). The Exchange Offer and the Second-Step Merger are not conditioned upon any financing arrangements or contingencies.

ACI has received a commitment letter from Wells Fargo, to arrange, and Wells Fargo Bank to provide, subject to certain conditions, up to $450 million for the purpose of financing a portion of the cash component of the consideration to be paid for each S1 Share, as well as for other payments made in connection with the Exchange Offer and to refinance ACI’s existing revolving facility. No other plans or arrangements have been made to finance or repay such financing after the consummation of the Exchange Offer and the Second-Step Merger. No alternative financing arrangements or alternative financing plans have been made in the event such financings fail to materialize. Please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Source and Amount of Funds.”

Non-Solicitation by S1 of S1 Acquisition Proposals

(See page 48)

The Transaction Agreement provides that, subject to limited exceptions, S1 will not, and will cause its subsidiaries not to and will use reasonable best efforts to cause its representatives not to, directly or indirectly initiate, solicit, knowingly encourage (including by way of furnishing non-public information), or take any other action designed to lead to, any inquiries or the making of any proposal that constitutes, or would reasonably be expected to lead to, the submission of any S1 Acquisition Proposal (as defined below at “The Transaction Agreement — Non-Solicitation by S1 of S1 Acquisition Proposals; Board Recommendation”) or engage, enter into, continue or participate in any negotiations or discussions with respect thereto or furnish any non-public information concerning S1 and its subsidiaries to any person in connection with any S1 Acquisition Proposal.

However, prior to the earlier of the Acceptance Time or the receipt of the approval of the merger of S1 and Offeror by the S1 stockholders, if S1 receives a written S1 Acquisition Proposal that the S1 Board believes in good faith is bona fide, and the S1 Board, after consultation with its financial advisors and outside legal counsel, determines in good faith that such S1 Acquisition Proposal constitutes or would reasonably be expected to lead to or result in an S1 Superior Offer (as defined below at “The Transaction Agreement — Non-Solicitation by S1; Board Recommendation”), then the S1 Board may, subject to certain conditions, furnish information with respect to S1 and participate in discussions with respect to such S1 Acquisition Proposal. If S1 receives a written S1 Acquisition Proposal that the S1 Board believes in good faith is bona fide, and the S1 Board, after consultation with its financial advisors and outside legal counsel, determines in good faith that such S1 Acquisition Proposal constitutes an S1 Superior Offer, then the S1 Board may at any time prior to the earlier of the Acceptance Time or the receipt of the approval of the Second-Step Merger by the S1 stockholders, if it determines in good faith, after consultation with outside counsel, that the failure to take such action would be inconsistent with its fiduciary duties, change its recommendation and/or, subject to certain conditions, cause S1 to terminate the Transaction Agreement and concurrently with such termination, upon payment of a termination fee in the amount of $19.14 million, enter into a definitive agreement with respect to such S1 Acquisition Proposal. The S1 Board may not change its recommendation and terminate the Transaction Agreement unless S1 has provided prior written notice to ACI of the reasons for such action at least five business days in advance of its taking such action, and during such notice period, S1 must negotiate with ACI in good faith and take into account all changes to the terms of the Transaction Agreement proposed by ACI in determining whether such S1 Acquisition Proposal continues to constitute an S1 Superior Offer. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Non-Solicitation by S1 of S1 Acquisition Proposals; Board Recommendation.”

Termination of the Transaction Agreement

(See page 53)

The Transaction Agreement may be terminated and the Exchange Offer and the Second-Step Merger may be abandoned:

Alternatives to the Exchange Offer

(See page 44)

ACI may only complete the Second-Step Merger if it purchases the S1 Shares pursuant to the Exchange Offer. However, the Transaction Agreement gives ACI the right to require that S1 convene a stockholders’ meeting to approve a merger in which the S1 stockholders would have the right to receive the Proration Amount of Cash and Stock Consideration as a result of a merger of Offeror and S1 instead of the Exchange Offer. The terms and conditions of such a transaction would be substantially the same as the terms and conditions of the Exchange Offer. ACI had not determined whether to exercise this right as of the date of this prospectus/offer to exchange. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Merger Without Meeting of Stockholders; Special Meeting.”

Termination Fees

(See page 53)

The Transaction Agreement provides that upon the termination of the Transaction Agreement under specified circumstances, S1 will owe ACI a cash termination fee of $19.14 million. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Termination Fees.”

Appraisal/Dissenters’ Rights

(See page 67)

No dissenters’ or appraisal rights are available in connection with the Exchange Offer. However, upon consummation of the Second-Step Merger, S1 stockholders who have not tendered their S1 Shares in the Exchange Offer and who, if a stockholder vote is required, do not vote for, or otherwise consent to, the approval of the Second-Step Merger will have rights under Delaware law to dissent from the Second-Step Merger and demand appraisal of their S1 Shares. Stockholders at the time of a “short form” merger under Delaware law would also be entitled to exercise dissenters’ rights pursuant to such a “short form” merger. Stockholders who perfect dissenters’ rights by complying with the procedures set forth in Section 262 of the DGCL will be entitled to receive a cash payment equal to the “fair value” of their S1 Shares, as determined by a Delaware court. Please see the section of this prospectus/offer to exchange titled “The Transaction Agreement — Appraisal Rights/Dissenting Shares.”

Material Federal Income Tax Consequences

(See page 63)

Based on closing trading prices of ACI Shares as of October 12, 2011, the Exchange Offer would be taxable to you because the integrated transaction would not qualify as a reorganization. If the integrated transaction does not qualify as a reorganization, you may be taxed on your exchange of S1 Shares for the Stock Consideration in the Exchange Offer or the Second-Step Merger depending on the surrounding facts. In general in this case, you will recognize a capital gain or a capital loss to the extent of the difference between your adjusted tax basis in your shares and the sum of the Cash Considerations and the fair market value of the Stock Consideration you receive.

If the Exchange Offer and the Second-Step Merger qualified as component parts of an integrated transaction that constitutes a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (the “Internal Revenue Code”), your exchange of S1 Shares for the Stock Consideration should be tax free, except to the extent that you also receive cash, as discussed below. Whether or not such transactions will so qualify is dependent on whether certain factual requirements are met, including that the Exchange Offer and Second-Step Merger are “interdependent” (that is, ACI would not undertake the Exchange Offer without the intention and expectation of completing the Second-Step Merger). In addition, there must be a “continuity of interest” of holders of S1 Shares in the combined company. ACI believes that this test should be satisfied if the total value of the Stock Consideration represents at least 40% of the total value of the consideration received by holders of S1 Shares, and may be satisfied at a slightly lower percentage. If market prices for ACI Shares upon consummation of the Exchange Offer are less than $41.48, the Stock Consideration would represent less than 40% of the total value of the Exchange Offer consideration. You are urged to obtain current trading price information prior to making any decision with respect to the Exchange Offer. We cannot provide any assurance as to whether these conditions will be satisfied at this time, since it may be affected, among other things, by the total value of the Stock Consideration at the time of the consummation of the Exchange Offer and the Second-Step Merger.

If the integrated transaction constitutes a reorganization within the meaning of Section 368(a) of the Internal Revenue Code, any gain (but not loss) you realize on the transaction will be treated as a taxable capital gain or dividend in an amount equal to the lesser of (1) the excess of the sum of the Cash Consideration and the fair market value of the Stock Consideration you receive in the transaction over your basis in your shares and (2) the amount of cash you receive in the transaction, including any cash you receive in lieu of a fractional ACI Share, depending on your circumstances. For more information, please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Material Federal Income Tax Consequences.”

THIS PROSPECTUS/OFFER TO EXCHANGE CONTAINS A GENERAL DESCRIPTION OF CERTAIN MATERIAL FEDERAL INCOME TAX CONSEQUENCES OF THE OFFER AND THE SECOND-STEP MERGER. THIS DESCRIPTION DOES NOT ADDRESS ANY NON-U.S. TAX CONSEQUENCES, NOR DOES IT PERTAIN TO STATE OR OTHER TAX CONSEQUENCES. CONSEQUENTLY, ACI URGES YOU TO CONTACT YOUR OWN TAX ADVISOR TO DETERMINE THE PARTICULAR TAX CONSEQUENCES TO YOU OF THE OFFER.

Accounting Treatment

(See page 81)

ACI will account for the acquisition of S1 Shares under the acquisition method of accounting for business transactions. ACI will be considered the acquirer of S1 for accounting purposes. In determining the acquirer for accounting purposes, ACI considered the factors required under the accounting principles generally accepted in the U.S., which is referred to as “U.S. GAAP.”

Regulatory Approval and Status

(See page 77)

U.S. Antitrust Clearance

The Exchange Offer is subject to review by the FTC and the Antitrust Division. Under the HSR Act, the Exchange Offer may not be completed until certain information has been provided to the FTC and the Antitrust Division and a required waiting period has expired or has been terminated.

ACI filed the required Notification and Report Form under the HSR Act with the Antitrust Division and the FTC on July 27, 2011. Thereafter, the Antitrust Division informed ACI that, as between the FTC and the Antitrust Division, the Antitrust Division would review ACI’s filing. ACI withdrew its initial filing on August 26, 2011, and refiled it on August 29, 2011 in order to permit the Antitrust Division to have additional time to review the filing. On September 27, 2011, ACI withdrew its initial HSR filing and refiled it on September 28, 2011 in order to permit the Antitrust Division to have additional time to review the filing. The 30-calendar day waiting period recommenced in connection with such refiling so that it now expires, unless terminated earlier or extended, at 11:59 p.m., Eastern time, on October 28, 2011. The Antitrust Division may extend its review beyond the 30-calendar day waiting period by requesting additional information and documentary material. In the event of such a request, the waiting period would be extended until 11:59 p.m., Eastern time, on the 30th calendar day after ACI has made a proper response to that request as specified by the HSR Act and the implementing rules.

We believe that the combination with S1 would provide ACI with enhanced scale, breadth and additional capabilities to compete more effectively in the highly competitive payment systems marketplace. If ACI were to acquire S1, we believe that the combined company would continue to face intense competition from third-party software vendors, in house solutions, processors, IT service organizations and credit card associations, including from companies which are substantially larger and have substantially greater market shares than the combined company would have. Moreover, we believe that the dynamic worldwide nature of the industry means that competitive alternatives can and do regularly emerge. Thus, ACI does not believe the transaction would enable it to obtain market power in, or even a significant share of, any relevant market. However, ACI has twice withdrawn and refiled its HSR Act filing prior to the date of this prospectus/offer to exchange in an effort to convince the DOJ staff of ACI’s view as to the competitive nature of payment systems marketplace, and there can be no assurance that the DOJ will concur with our belief. If ACI again withdraws and refiles its HSR Act filing, the DOJ issues a request for additional information or documentary material or the DOJ institutes an action challenging the transaction, the Expiration Time would be extended and the completion of the Exchange Offer could be prevented.

In the Transaction Agreement, each of ACI and S1 has agreed to, among other things, use its reasonable best efforts to obtain promptly, and no later than July 31, 2012, obtain any clearance required under the antitrust laws and to avoid any impediment under the antitrust laws. If necessary to avoid the entry of an injunction sought or

issued by the Antitrust Division (a “DOJ Impediment”), ACI’s covenants under the Transaction Agreement include ACI being required under the Transaction Agreement to offer to the DOJ that it or its subsidiaries take, and, if such offer is accepted by the DOJ, use its best efforts to eliminate any DOJ Impediment. For this purpose, ACI’s “best efforts to eliminate any DOJ Impediment” as set forth in the immediately preceding sentence will require that ACI use its best efforts to effect such of the following as may be necessary to avoid a DOJ Impediment: (1) the sale, holding separate, licensing, modifying or otherwise disposing of all or any portion of the business, assets or properties of S1 or its subsidiaries, whether located in or outside the United States; (2) ACI conducting or limiting the conduct of the business, assets or properties of S1 or its subsidiaries, whether located in or outside the United States, in a specified manner; or (3) S1 or its subsidiaries’ entry with the DOJ into any agreement, settlement, order, other relief or action of a type referred to in clause (2).

Other Regulatory Approvals

The Exchange Offer and the Second-Step Merger will also be subject to review by antitrust and other authorities in jurisdictions outside the U.S. ACI is in the process of filing as soon as practicable all applications and notifications determined by ACI to be necessary or advisable under the laws of the respective jurisdictions for the consummation of the Exchange Offer and the Second-Step Merger.

For more information, please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Certain Legal Matters; Regulatory Approvals.”

Listing of ACI Shares to be Issued Pursuant to the Exchange Offer and the Second-Step Merger

(See page 52)

ACI will submit the necessary applications to cause the ACI Shares to be issued as the Stock Consideration of the Exchange Offer and the Second-Step Merger to be authorized for listing on the NASDAQ Global Select Market.

Comparison of Stockholders’ Rights

(See page 84)

You may receive ACI Shares as a portion of the Exchange Offer consideration, subject to your election and proration. Because there are a number of differences between the rights of a stockholder of S1 and the rights of a stockholder of ACI, ACI urges you to review the discussion in the section of this prospectus/offer to exchange titled “Comparison of Stockholders’ Rights.”

Expiration Time of the Exchange Offer

(See page 55)

The Exchange Offer is scheduled to expire at 5:00 p.m., Eastern time, on Monday, October 31, 2011, which is the Expiration Time, unless further extended by Offeror. For more information, you should read the discussion in the section of this prospectus/offer to exchange titled “The Exchange Offer — Extension, Termination, Waiver and Amendment.”

Extension, Termination, Waiver and Amendment

(See page 56)

Transaction Agreement provides that Offeror will extend the Expiration Time from time to time, including as follows:

in each case, by making public announcement thereof.

The Expiration Time may be subject to multiple extensions and any decision to extend the Exchange Offer will be made prior to the Expiration Time. Additionally, Offeror may elect to provide a subsequent offering period of at least three business days following the Expiration Time.

Subject to the applicable rules of the SEC and the terms and conditions of the Exchange Offer, Offeror expressly reserves the right (but shall not be obligated) at any time or from time to time in its sole discretion to waive any Exchange Offer condition or modify or amend the terms of the Exchange Offer, except that, without the prior written consent of S1,

For more information, please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Extension, Termination, Waiver and Amendment.”

Procedure for Tendering Shares

(See page 59)

The procedure for tendering S1 Shares varies depending on whether you possess physical certificates, a nominee holds your certificates for you, or whether you or a nominee hold your S1 Shares in book-entry form. ACI urges you to read the section of this prospectus/offer to exchange titled “The Exchange Offer — Procedure for Tendering” as well as the transmittal materials, including the letter of election and transmittal.

Withdrawal Rights

(See page 62)

You can withdraw tendered S1 Shares at any time until the Exchange Offer has expired. If Offeror decides to provide a subsequent offering period, it will accept S1 Shares validly tendered during that period immediately and you will not be able to withdraw shares tendered in the Exchange Offer during any subsequent offering period. Please see the section of this prospectus/offer to exchange titled “The Exchange Offer — Withdrawal Rights.”

Acceptance for Exchange and Exchange of S1 Shares; Delivery of Exchange Offer Consideration

(See page 57)

Upon the terms and subject to the conditions of the Exchange Offer (including, if the Exchange Offer is extended or amended, the terms and conditions of any such extension or amendment), Offeror will accept for exchange, and will exchange for ACI Shares and cash promptly after the Expiration Time, all S1 Shares validly tendered and not properly withdrawn. If Offeror elects to provide a subsequent offering period following the Expiration Time, S1 Shares validly tendered during such subsequent offering period will be accepted for exchange immediately upon tender and will be promptly exchanged for the Exchange Offer consideration. For more information, please see the section of this prospectus/offer to exchange under the caption titled “The Exchange Offer — Acceptance for Exchange and Exchange of S1 Shares; Delivery of Exchange Offer Consideration.”

Cash in Lieu of Fractional ACI Shares

(See page 58)

Certificates representing fractional ACI Shares will not be distributed in the Exchange Offer or the Second-Step Merger. Instead, each tendering S1 stockholder who would otherwise be entitled to a fractional ACI Share will receive cash (rounded to the nearest whole cent) in an amount (without interest) equal to the product of (1) such fraction, multiplied by (2) the volume weighted average sales price per share of ACI Shares for the ten consecutive days that ACI Shares have traded ending on and including the second clear trading day immediately prior to the Acceptance Time or Effective Time, as applicable, as reported on the NASDAQ.

Elections and Proration

(See page 58)

S1 stockholders may elect to receive the Stock Consideration or the Cash Consideration in exchange for each S1 Share validly tendered and not withdrawn pursuant to the Exchange Offer, subject, in the case of elections of the Cash Consideration or the Stock Consideration, to the proration procedures described in this prospectus/offer to exchange and the related letter of election and transmittal, by indicating their elections in the applicable section of the letter of election and transmittal. If an S1 stockholder decides to change its election after tendering its S1 Shares, such S1 stockholder must first properly withdraw the tendered S1 Shares and then retender the S1 Shares prior to the Expiration Time, with a new letter of election and transmittal that indicates the revised election. S1 stockholders who do not make an election will be deemed to have elected the Cash Consideration.

S1 stockholders electing either the Cash Consideration or the Stock Consideration will be subject to proration so that 66.2% of S1 Shares will be exchanged for the Cash Consideration and 33.8% of S1 Shares will be exchanged for the Stock Consideration in the Exchange Offer. S1 stockholders who do not participate in the Exchange Offer and whose shares are acquired in the Second-Step Merger will receive the Proration Amount of Cash and Stock Consideration. The elections of other S1 stockholders will affect whether a tendering S1 stockholder electing the Cash Consideration or the Stock Consideration receives solely the type of consideration elected or if a portion of such S1 stockholder’s tendered S1 Shares is exchanged for another form of consideration. S1 stockholders who otherwise would be entitled to receive a fractional ACI Share will instead receive cash in lieu of any fractional ACI Share such holder may have otherwise been entitled to receive.

Risk Factors

(See page 25)

The Exchange Offer and the Second-Step Merger are, and if the Exchange Offer and the Second-Step Merger are consummated, the combined company will be, subject to several risks which you should carefully consider prior to participating in the Exchange Offer.

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF ACI

Set forth below is certain selected historical consolidated financial data relating to ACI. The financial data has been derived from ACI’s Quarterly Report on Form 10-Q for the six months ended June 30, 2011, which is incorporated by reference into this prospectus/offer to exchange (the “ACI 10-Q”) and ACI’s Annual Report on Form 10-K for the year ended December 31, 2010, which is incorporated by reference into this prospectus/offer to exchange (the “ACI 10-K”). You should not take historical results as necessarily indicative of the results that may be expected for the remainder of this fiscal year or any other future period. This financial data should be read in conjunction with the financial statements and the related notes and other financial information contained in the ACI 10-Q and the ACI 10-K. More comprehensive financial information, including “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” is contained in the ACI 10-Q and ACI 10-K, and the following summary is qualified in its entirety by reference to the ACI 10-Q and ACI 10-K and all of the financial information and notes contained therein. Please see the section of the prospectus/offer to exchange titled “Where You Can Find More Information.”

The following table sets forth selected historical consolidated financial data for the years ended December 31, 2010, 2009 and 2008, the three months ended December 31, 2007 and the years ended September 30, 2007 and 2006 and the six months ended June 30, 2011 and June 30, 2010:

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF S1

Set forth below is certain selected historical consolidated financial data relating to S1. The financial data has been derived from S1’s Quarterly Report on Form 10-Q for the six months ended June 30, 2011 (the “S1 10-Q”), which is incorporated by reference into this prospectus/offer to exchange, and S1’s Annual Report on Form 10-K for the year ended December 31, 2010 (the “S1 10-K”), which is incorporated by reference into this prospectus/offer to exchange. You should not take historical results as necessarily indicative of the results that may be expected for any future period. This financial data should be read in conjunction with the financial statements and the related notes and other financial information contained in the S1 10-Q and the S1 10-K. More comprehensive financial information, including “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” is contained in other documents filed by S1 with the SEC, and the following summary is qualified in its entirety by reference to such other documents and all of the financial information and notes contained in those documents. Please see the section of this prospectus/offer to exchange titled “Where You Can Find More Information.”

The following table sets forth selected historical consolidated financial data for the years ended December 31, 2010, 2009, 2008, 2007 and 2006 and the six months ended June 30, 2011 and June 30, 2010:

SUMMARY SELECTED UNAUDITED PRO FORMA COMBINED FINANCIAL INFORMATION

The following summary selected unaudited pro forma combined financial information has been prepared to illustrate the effect of the combination of ACI and S1 and has been prepared for informational purposes only. The unaudited pro forma combined balance sheet information combines information from the historical consolidated balance sheets of ACI and of S1 as of June 30, 2011, giving effect to the acquisition of S1 by ACI as if it had occurred on June 30, 2011. The unaudited pro forma combined statements of operations information combines information from the historical consolidated statements of operations of ACI and of S1 for the year ended December 31, 2010 and the six months ended June 30, 2011, giving effect to the acquisition of S1 by ACI as if it had occurred on January 1, 2010. The summary selected unaudited pro forma combined financial information has been prepared using the acquisition method of accounting under U.S. GAAP. ACI has been treated as the acquirer for accounting purposes.

The summary selected unaudited pro forma combined financial information has been presented for informational purposes only. The pro forma information is not necessarily indicative of what the combined company’s financial position or results of operations actually would have been had the acquisition been completed as of the dates indicated. In addition, the summary selected unaudited pro forma combined financial information does not purport to project the future financial position or operating results of the combined company. The following information has been derived from, and should be read in conjunction with, the unaudited pro forma condensed combined financial information and related notes included in this prospectus/offer to exchange. See “Unaudited Condensed Combined Pro Forma Financial Information.”

This pro forma information is subject to risks and uncertainties, including those discussed in the section of this prospectus/offer to exchange titled “Risk Factors.”

The following sets forth unaudited summarized pro forma statement of operations data for the six months ended June 30, 2011 and the year ended December 31, 2010 (in thousands of dollars):

The following sets forth unaudited summarized pro forma balance sheet data as of June 30, 2011 (in thousands of dollars):