Exhibit 4.2

ASSUMPTION OF WARRANT AGREEMENT

THIS ASSUMPTION OF WARRANT AGREEMENT (this “Agreement”), is made and entered into effective as of October 21, 2026, by and among Archimedes Tech SPAC Partners II Co., a Cayman Islands exempted company (the “Company”), ATII Holdings Inc. (to be renamed as Forge Nano Holdings, Inc.), a Delaware corporation (“Pubco”), and Odyssey Transfer and Trust Company, a Minnesota corporation, as warrant agent (the “Warrant Agent”). Capitalized terms used herein but not otherwise defined shall have the meanings given to them in the Warrant Agreement (as defined below).

WHEREAS, Archimedes Tech SPAC Partners II Co., a Cayman Islands exempted company (the “Company”), and the Warrant Agent previously entered into that certain Warrant Agreement, dated February 10, 2025 (the “Warrant Agreement”), pursuant to which the Warrant Agent agreed to act on behalf of the Company in connection with the issuance, registration, transfer, exchange, redemption and exercise of, among other Warrants, up to 11,500,000 warrants (the “Public Warrants”) underlying units issued in the Company’s initial public offering (the “IPO”), up to 420,000 warrants (the “Private Warrants”) issued in a private placement consummated simultaneously with the IPO, and up to 75,000 Working Capital Warrants issuable upon conversion of working capital loans;

WHEREAS, the IPO and the private placement were consummated on February 12, 2025;

WHEREAS, the Company has entered into an agreement and plan of merger, dated as of April 20, 2026 (as it may be amended from time to time, the “Merger Agreement”), by and among the Company, Pubco, ATII Merger Sub Inc., a Delaware corporation and a wholly-owned subsidiary of the Company (“Merger Sub I”), ATII Merger Sub II, LLC, a Delaware limited liability company and a wholly owned subsidiary of the Company (“Merger Sub II”), and Forge Nano, Inc., a Delaware corporation (“Forge Nano”), which provides for a Business Combination between the Company and Forge Nano;

WHEREAS, pursuant to the Merger Agreement, (i) the Company will merge with and into Pubco, upon which the separate existence of the Company will cease and Pubco will be the surviving corporation (the “Domestication Merger”) and (ii) following the Domestication Merger, (A) Merger Sub I will merge with and into Forge Nano (the “First Company Merger”), with Forge Nano continuing as the surviving corporation in the First Company Merger and a wholly-owned subsidiary of Pubco (the “Initial Surviving Corporation”), and (B) immediately following the effective time of the First Company Merger, the Initial Surviving Corporation will merge with and into Merger Sub II (the “Second Company Merger”), with Merger Sub II continuing as the surviving company in the Second Company Merger and as a wholly-owned subsidiary of Pubco; and as a result of the Business Combination, holders of Ordinary Shares of the Company and equity interests of Forge Nano will become holders of shares of common stock, par value $0.0001 per share, of Pubco (“Pubco Common Stock”);

WHEREAS, upon consummation of the Business Combination, as provided in Section 4.5 of the Warrant Agreement, the Warrants will no longer be exercisable for Ordinary Shares of the Company but instead will be exercisable for Pubco Common Stock; and

WHEREAS, in connection with the Business Combination, Pubco wishes to confirm the assumption of the Company’s rights, duties, covenants and other obligations (the “Obligations”) under the Warrant Agreement;

NOW, THEREFORE, for good and valuable consideration, the receipt and adequacy of which are hereby acknowledged, the parties agree as follows:

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IN WITNESS WHEREOF, the parties have executed this Agreement as of the date first above written.