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                                                                     Exhibit 4.5

                       FIRST AMENDMENT TO RIGHTS AGREEMENT

                  THIS FIRST AMENDMENT TO RIGHTS AGREEMENT (this "Amendment") is
entered into as of December 3, 2001 by and between HARRIS TRUST COMPANY OF
CALIFORNIA (hereinafter referred to as the "Rights Agent"), and AVANT!
CORPORATION (hereinafter referred to as the "Company"). Capitalized terms not
otherwise defined herein shall have the meanings set forth in the Rights
Agreement (defined below).

                  WHEREAS, the Rights Agent and the Company are parties to that
certain Rights Agreement, effective as of September 4, 1998 (the "Rights
Agreement"); and

                  WHEREAS, pursuant to Sections 27 and 34 of the Rights
Agreement, on December 3, 2001, the Board of Directors of the Company amended
the Rights Agreement as described herein; and

                  WHEREAS, pursuant to Section 27 of the Rights Agreement, such
amendment shall be evidenced by a writing signed by the Company and the Rights
Agent

                  NOW, THEREFORE, the Rights Agreement is hereby amended as
follows:

1. DEFINITION OF ACQUIRING PERSON. The definition of "Acquiring Person" in
Section 1 (Certain Definitions) of the Rights Agreement is amended and restated
in its entirety to read as follows:


                  "'Acquiring Person' shall mean any Person who or which,
         together with all Affiliates and Associates of such Person, shall be
         the Beneficial Owner of 15%, or in the case of (i) a Grandfathered
         Stockholder other than a Second Tier Grandfathered Stockholder, 20%, or
         (ii) a Second Tier Grandfathered Stockholder, the greater of 15% or
         such percentage as is beneficially owned by each Amerindo Holder plus
         1% or more of the Common Shares of the Company then outstanding but
         shall not include (i) the Company, (ii) any Subsidiary of the Company,
         (iii) any employee benefit plan of the Company or any Subsidiary of the
         Company, (iv) any entity holding Common Shares for or pursuant to the
         terms of any such employee benefit plan or (v) any Person designated by
         the Board of Directors of the Company as an "Exempt Person," unless and
         until such Person shall thereafter become the Beneficial Owner of
         additional shares constituting 1% or more of the Common Shares of the
         Company then outstanding otherwise than in a transaction or series of
         transactions approved prior to such transaction or transactions by the
         Board of Directors of the Company. Notwithstanding the foregoing, (1)
         no Person shall become an `Acquiring Person' as the result of an
         acquisition of Common Shares by the Company which, by reducing the
         number of shares outstanding, increases the proportionate number of
         shares beneficially owned by such Person to 15% (or such other
         percentage as would otherwise result in such person becoming an
         Acquiring Person) or more of the Common Shares of the Company then
         outstanding; PROVIDED, HOWEVER, that if a Person shall so become the
         Beneficial Owner of 15% (or such other percentage) or more of the
         Common Shares of the Company then outstanding by reason of an
         acquisition of Common Shares by the Company and shall, after such share
         purchase by the Company, become the Beneficial Owner of an additional
         1% of the outstanding Common Shares of the Company, then such Person
         shall be deemed to be an `Acquiring


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         Person'; and (2) if the Board of Directors of the Company determines in
         good faith that a Person who would otherwise be an `Acquiring Person,'
         as defined pursuant to the foregoing provisions of this paragraph, has
         become such inadvertently, and such Person divests as promptly as
         practicable a sufficient number of Common Shares so that such Person
         would no longer be an `Acquiring Person,' as defined pursuant to the
         foregoing provisions of this paragraph, then such Person shall not be
         deemed to have become an `Acquiring Person' for any purposes of this
         Agreement."

2. DEFINITION OF FINAL EXPIRATION DATE. The definition of "Final Expiration
Date" in Section 1 (Certain Definitions) of the Rights Agreement is amended and
restated in its entirety to read as follows:


                  "'Final Expiration Date' shall mean the earlier to occur of
         (i) the date and time immediately prior to the effective time (the
         "Effective Time") of the merger (the "Merger") of the Company with and
         into Maple Forest Acquisition L.L.C., a Delaware limited liability
         company (the "Merger Sub"), as contemplated by that certain Agreement
         and Plan of Merger, dated as of December 3, 2001, among the Company,
         Synopsys, Inc. and Merger Sub (the "Merger Agreement") (in which case,
         for purposes of the Agreement and notwithstanding any provision to the
         contrary therein, the "Close of Business on the Final Expiration Date"
         shall mean the Effective Time), and (ii) September 4, 2008.'"

3. TERMINATION OF THE RIGHTS AGREEMENT. Notwithstanding any provision in this
Amendment or the Rights Agreement to the contrary, the Rights Agreement shall
terminate and shall be of no force or effect immediately prior to the Effective
Time of the Merger. If the Merger Agreement has been terminated prior to
consummation of the Merger, the Rights Agreement shall not terminate and shall
continue in full force and effect in accordance with its terms.

4. GENERAL PROVISIONS.


         (a) Successors and Assigns. The provisions of this Amendment by or for
the benefit of the Company or the Rights Agent shall bind and inure to the
benefit of their respective successors and assigns hereunder.

         (b) Counterparts. This Amendment may be executed in any number of
counterparts and each of such counterparts shall for all purposes be deemed to
be an original, and all such counterparts shall together constitute but one and
the same instrument.

         (c) Governing Law. This Amendment shall be governed by and construed in
accordance with the laws of the State of Delaware applicable to contracts to be
made and performed entirely within such State.


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         (d) Severability. If any term, provision, covenant or restriction of
this Amendment is held by a court of competent jurisdiction or other authority
to be invalid, void or unenforceable, the remainder of the terms, provisions,
covenants and restrictions of this Amendment shall remain in full force and
effect and shall in no way be affected, impaired or invalidated.


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         IN WITNESS WHEREOF, the parties have executed this Amendment on the
date first hereinabove stated.

COMPANY:                                    RIGHTS AGENT:

AVANT! CORPORATION                          HARRIS TRUST COMPANY OF CALIFORNIA,
                                            as Rights Agent

By: /s/ Scott Spangenberg                   By:
                                               ---------------------------------

Its: Corporate Controller                   Its:
                                               ---------------------------------



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