Exhibit 8.1

Altimar Acquisition Corp. II

40 West 57th Street, 33rd Floor

New York, New York 10019

Re:     United States Federal Income Tax Considerations

Ladies and Gentlemen:

We have acted as United States federal income tax counsel for Altimar Acquisition Corp. II, a Cayman Islands company (the “Company”), in connection with the Business Combination Agreement, dated as of July 15, 2021 (the “Business Combination Agreement”), by and among the Company, Fathom Holdco, LLC, Rapid Merger Sub, LLC, Rapid Blocker 1 Merger Sub, LLC, Rapid Blocker 2 Merger Sub, LLC, Rapid Blocker 3 Merger Sub, LLC, CORE Fund I Blocker-5 LLC, CORE Fund I Blocker-2 LLC and SG (MCT) Blocker, LLC, which, among other things, provides for the Company’s domestication from a Cayman Islands exempted company to a Delaware corporation pursuant to Section 338 of the Delaware General Corporation Law, as amended, and Article 206 of the Cayman Islands Companies Law (2021 Revision) (the “Domestication”). This opinion is being delivered in connection with the Registration Statement (File No. 333-259639) of the Company on Form S-4 filed on September 20, 2021 with the Securities and Exchange Commission, as amended and supplemented

through the date hereof (the “Registration Statement”). Capitalized terms not defined herein have the meanings specified in the Business Combination Agreement unless otherwise indicated.

In connection with this opinion, we have examined and relied on originals or copies, certified or otherwise identified to our satisfaction, of (i) the Business Combination Agreement, (ii) the Registration Statement, (iii) the representation letter of the Company delivered to us for purposes of this opinion (the “Representation Letter”), and (iv) such other documents, certificates and records as we have deemed necessary or appropriate as a basis for the opinion set forth herein. We have not, however, undertaken any independent investigation of any factual matter set forth in any of the foregoing.

In rendering our opinion, we have assumed, without any independent investigation or examination thereof, that (i) the Domestication will be consummated in the manner described in the Registration Statement and the business combination will be consummated in the manner described in the Registration Statement and the Business Combination Agreement, each will be effective under applicable law, and none of the terms or conditions contained in either the Registration Statement or the Business Combination Agreement will be waived or modified, (ii) the facts relating to (A) the Domestication are accurately and completely reflected in the Registration Statement and (B) the business combination are accurately and completely reflected in the Registration Statement and the Business Combination Agreement, (iii) the Business Combination Agreement, Registration Statement and all documents described therein represent the entire understanding between the parties to the Business Combination Agreement with respect to the Domestication and related transactions, (iv) any representations made in the Representation Letter are true, complete and correct and will remain true, complete and correct at all times up to and including the effective time of the Domestication, (v) any representations made in the Representation Letter subject to qualification relating to the knowledge, belief, expectation or intent of any party are true, complete and correct and will remain true, complete and correct at all times up to and including the effective time of the Domestication, in each case, without such qualification and (vi) the Company will not take any position on any federal, state, or local income or franchise tax return, or take any other tax reporting position that is inconsistent with this opinion. Our opinion assumes and is expressly conditioned on, among other things, the initial and continuing accuracy of the facts, information, covenants, representations and warranties set forth in the documents referred to above.

For purposes of our opinion, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed, photostatic or electronic copies, and the authenticity of the originals of such latter documents. We have assumed that the Business Combination Agreement and such other documents, certificates, and records are, and will continue to be, duly authorized, valid, and enforceable.

The opinion set forth below is based on the Internal Revenue Code of 1986, as amended (the “Code”), administrative rulings, judicial decisions, Treasury regulations

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and other applicable authorities, all as in effect on the effective date of the Registration Statement. The statutory provisions, regulations, and interpretations upon which our opinion is based are subject to change, and such changes could apply retroactively. Any change in law or the facts regarding the Domestication or any of the transactions related thereto, or any inaccuracy in the facts or assumptions on which we relied, could affect the continuing validity of the opinion set forth below. We assume no responsibility to inform you of any such changes or inaccuracy that may occur or come to our attention. The opinion set forth herein has no binding effect on the United States Internal Revenue Service (“IRS”) or the courts of the United States. No assurance can be given that, if the matter were contested, a court would agree with the opinion set forth herein.

Based upon the foregoing and subject to the assumptions, exceptions, limitations and qualifications set forth herein and in the Registration Statement under the heading “Material U.S. Federal Income Tax Considerations,” we are of the opinion that, for United States federal income tax purposes, the Domestication should qualify as a “reorganization” within the meaning of Section 368(a)(1)(F) of the Code. We express no opinion on any other potential U.S. federal income tax consequences of the Domestication (including tax consequences pursuant to Section 367 of the Code or the passive foreign investment company rules).

Except as set forth above, we express no opinion as to the tax consequences to any party, whether federal, state, local or foreign, of any transactions related to the Domestication or contemplated by the Business Combination Agreement and this opinion may not be relied upon except with respect to the consequences specifically discussed herein. Furthermore, our opinion is based on current U.S. federal income tax law and administrative practice, and we do not undertake to advise you as to any changes after the date hereof of the Domestication in U.S. federal income tax law that may affect our opinion. In addition, there can be no assurance that changes in the law will not take place which could affect the U.S. federal income tax consequences of the Domestication or that contrary positions may not be taken by the IRS. To the extent any of the representations, warranties, statements and assumptions material to our opinion and upon which we have relied are not accurate and complete in all material respects at all the relevant times, our opinion would be adversely affected and should not be relied upon.

We are furnishing this letter in our capacity as United States federal income tax counsel to the Company. This opinion has been prepared in connection with the filing of the Registration Statement and may not be relied upon for any other purpose without our prior written consent.

We hereby consent to use of this opinion as an exhibit to the Registration Statement and to the use of our name under the caption “Material U.S. Federal Income Tax Considerations” and “Legal Matters” contained in the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules and regulations thereunder.

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Very truly yours,

/s/ Paul, Weiss, Rifkind, Wharton & Garrison LLP

PAUL, WEISS, RIFKIND, WHARTON & GARRISON LLP

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