<PAGE>
                                                                    EXHIBIT 10.5


                        AMENDMENT TO EMPLOYMENT AGREEMENT

         This Amendment to Employment Agreement is entered into as of December
3, 2001 (the "Effective Date") by and between Avant! Corporation (the "Company")
and Sheng-Chun Lo (the "Employee").

         WHEREAS, the Company has entered into an employment agreement with the
Employee dated October 1, 2000 (the "Employment Agreement"); and

         WHEREAS, the Company has entered into a Merger Agreement, dated as of
the date hereof among the Company, Synopsys, Inc. ("Synopsys") and certain other
parties (the "Merger Agreement"); and

         WHEREAS, the Company and the Employee wish to acknowledge and reaffirm
the continued effectiveness of the Employment Agreement as amended hereby.

         NOW THEREFORE, the parties hereto hereby agree as follows:

         1. In General. The Company and the Employee acknowledge and reaffirm,
effective as of the date hereof, all of the terms and conditions, including the
Employee's obligations, under the Employment Agreement, subject only to such
changes as are expressly provided for herein. For the benefit of the Company and
Synopsys, the Employee acknowledges that his reaffirmation of his agreements in
Section 3 of the Employment Agreement is a material inducement to Synopsys to
enter into the Merger Agreement. Further, the Employee acknowledges that the
Term (as defined in the Employment Agreement) and the provisions of Section 1(c)
of the Employment Agreement are not affected by this Amendment. The Employee
agrees not to take any position, or to assert any defense, in connection with
any enforcement of those provisions that is inconsistent with such
acknowledgements.

         2. Section 3(A). Section 3(A) of the Employment Agreement will be
revised to read in its entirety as follows:

                "(A) NON-COMPETITION.

                The parties confirm that it is reasonably necessary for the
                protection of the Company that the Employee agree, and
                accordingly, the Employee does hereby agree that he will not,
                directly or indirectly, at any time during his employment
                hereunder and thereafter during the Restricted Period, as
                hereinafter defined, engage in any aspect of the Company
                Business (as hereinafter defined) other than as an employee of
                the Company.

                  (i) The Employee shall be deemed to be engaging in Company
                Business if he:

                       (1) directly or indirectly, whether or not for
                  compensation, participates in the ownership, management,
                  operation or control of any Competitor (as hereinafter
                  defined) or is employed by or performs consulting services for
                  any Competitor;

                       (2) directly or indirectly, solicits any customer of the
                  Company or any prospective customer of the Company while the
                  Employee was employed by the Company, with a view to inducing
                  such customer or prospective customer to enter into an
                  agreement or otherwise do business with any Competitor with
                  respect to Company Business, or attempts
<PAGE>
                  to induce any such customer or prospective customer to
                  terminate its relationship with the Company or to not enter
                  into a relationship with the Company, as the case may be;

                       (3) releases any customer or prospect lists of the
                  Company, or any other documents or other information (whether
                  or not such information is in writing) proprietary to the
                  Company or any customer of the Company, or otherwise
                  confidential or non-public, to any person, except with the
                  Company's written consent or as may be required pursuant to
                  the order of a court of competent jurisdiction; or

                       (4) offers employment to any employee of the Company
                  or attempts to induce any such employee to leave the employ of
                  the Company.

                  (ii) For purposes of this Section 3:

                       (1) The "Company Business" is any business in which the
                  Company is engaged or is actively proposed to be engaged at
                  the Effective Time or within six months preceding the
                  Effective Time.

                       (2) A "Competitor" is any corporation, firm, partnership,
                  proprietorship or other entity which engages in a Specified
                  Area (as hereinafter defined) in the Company Business.

                       (3) A "Specified Area" is any geographical area in which
                  the Company is engaged in the Company Business, including any
                  geographic area in which the Company is engaged in research
                  and development related to, or the production or sale of,
                  products related to the Company Business.

                  (iii) The parties hereto hereby declare that it is impossible
                  to measure in money the damages which will accrue to the
                  Employer by reason of a failure by the Employee to perform any
                  of his obligations under this Section 3(A). Accordingly, if
                  the Company institutes any action or proceeding to enforce
                  such obligations, to the extent permitted by applicable law,
                  the Employee hereby waives the claim or defense that the
                  Company has an adequate remedy at law, and the Employee shall
                  not urge in any such action or proceeding the claim or defense
                  that any such remedy at law exists.

                  (iv) The restrictions in this Section 3 shall be in addition
                  to any restrictions imposed on the Employee by statute or at
                  common law.";

         3. Section 3(C). The second paragraph of Section 3(C) will be deleted
in its entirety and the third paragraph of Section 3(C) will be revised to
change all references to "paragraph (a)" to "paragraph (A)";

         4. Section 4. Section 4 of the Employment Agreement is revised to read
in its entirety as follows:

                  4.  RELEASE AFTER TERMINATION OF EMPLOYMENT.

                  "Upon the termination of the Employee's employment, the
                  Company and the Employee agree that in consideration for the
                  Employee's services and for the consideration paid to Employee
                  by the Company, the Company (or its successor) and Employee
                  shall execute general releases in the forms attached hereto as
                  Exhibits A and B."


                                       2
<PAGE>
         5. Exhibit A. Exhibit A to this Amendment to Employment Agreement shall
be attached as Exhibit A to the Employment Agreement;

         6. Exhibit B. Exhibit B to this Amendment to Employment Agreement shall
be attached as Exhibit B to the Employment Agreement;

         7. Section 6. Section 6 of the Employment Agreement will be revised to
read in its entirety as follows:

                  "The Employee agrees and acknowledges that the Proprietary
                  Information and Inventions Agreement executed by him continues
                  in full force and effect in accordance with its terms."; and

         8. Section 7(M). Section 7(M) of the Employment Agreement will be
revised by replacing the first word of the first sentence with the following
language:

                  "Except for any controversy or claim between the Company and
                  Employee under Section 3 of this Employment Agreement, any"

         9. Effectiveness of Amendment. This Amendment shall be effective as of
the Effective Date; provided, however, that in the event the Merger Agreement
shall be terminated as provided therein, this Amendment shall be void and of no
further effect and the Employment Agreement shall be interpreted and construed
without giving effect to this Amendment.


                                       3
<PAGE>
         IN WITNESS WHEREOF, the Company and the Employee have executed this
Amendment to Employment Agreement, as of the date first above written.

                                                  AVANT! CORPORATION



                                                   /s/ Sheng-Chun Lo
                                                  ----------------------------
                                                  By:    Sheng-Chun Lo
                                                  Title: President



                                                  Sheng-Chun Lo



                                                   /s/ Sheng-Chun Lo
                                                  ----------------------------


                                       4
<PAGE>
                                    EXHIBIT A

         SYNOPSYS, INC., on behalf of itself, its corporate parents,
subsidiaries and affiliates, including without limitation Avant! Corporation,
and its present and former directors, managing directors, officers, control
persons, stockholders, employees, agents, attorneys, administrators, successors
and assigns (collectively, "RELEASOR"), for good and valuable consideration
received from the Employee, releases and discharges the Employee and his heirs,
executors, agents, attorneys, administrators, successors and assigns
(collectively, "RELEASEE") from any claim or cause of action, accounts,
agreements, bonds, bills, covenants, contracts, controversies, claims, damages,
demands, debts, dues, extents, executions, judgments, liabilities, obligations,
promises, predicate acts, reckonings, specialties, suits, sums of money,
trespasses and variances, including without limitation court costs and attorneys
fees, in law or in equity, which RELEASOR has as of the Effective Date against
RELEASEE that are based on actions or omissions of RELEASEE occurring prior to
the Effective Date which, as of the Effective Date (A) are known to RELEASOR,
(B) have been disclosed in a publicly available report or other document filed
on or before the Effective Date with the U.S. Securities and Exchange Commission
by or on behalf of RELEASEE or the Company, or (C) have been disclosed to
RELEASOR by RELEASEE or the Company on or before the Effective Date, the
information described in (A), (B) and (C) being hereinafter referred to as the
"Disclosed Facts" (collectively, the "RELEASED MATTERS"). For the purposes of
this paragraph, a claim against RELEASEE shall be deemed to have existed as of
the Effective Date if such claim is based upon actions or omissions of RELEASEE
that occurred or began prior to the Effective Date, even if (i) RELEASOR' s
right to damages or equitable relief on such claim had not matured as of the
Effective Date; or (2) RELEASOR became entitled to additional or different legal
or equitable relief on such claim after Effective Date as a result of a
continuation of the same actions or omissions or the effects thereof, and/or the
passage of time, after the Effective Date, and such claim shall be deemed to
include the right to any such additional or different relief. THE PARTIES
ACKNOWLEDGE THAT THEY ARE FAMILIAR WITH SECTION 1542 OF THE CIVIL CODE OF THE
STATE OF CALIFORNIA, WHICH PROVIDES AS FOLLOWS:

         "A GENERAL RELEASE DOES NOT EXTEND TO CLAIMS WHICH THE CREDITOR DOES
NOT KNOW OR SUSPECT TO EXIST IN HIS FAVOR AT THE TIME OF EXECUTING THE RELEASE,
WHICH IF KNOWN BY HIM MUST HAVE MATERIALLY AFFECTED HIS SETTLEMENT WITH THE
DEBTOR."

         RELEASOR EXPRESSLY WAIVES AND RELINQUISHES ANY RIGHT OR BENEFIT WHICH
IT HAS OR MAY HAVE UNDER SECTION 1542 OF THE CIVIL CODE OF THE STATE OF
CALIFORNIA, OR ANY OTHER STATUTE OR LEGAL PRINCIPLE WITH SIMILAR EFFECT WITH
RESPECT TO CLAIMS BASED ON OR ARISING FROM THE DISCLOSED FACTS, BUT RELEASOR
DOES NOT WAIVE OR RELINQUISH ANY SUCH RIGHT OR BENEFIT WITH RESPECT TO CLAIMS
THAT ARE NOT BASED ON OR DO NOT ARISE FROM THE DISCLOSED FACTS.

         RELEASEE acknowledges that he is aware that, after executing this
RELEASE, RELEASOR or its attorneys or agents may discover facts in addition to
or different from the Disclosed Facts, and that it is the intention of RELEASOR
and RELEASEE not to settle and release any claim or cause of action, accounts,
agreements, bonds, bills, covenants, contracts, controversies, claims, damages,
demands, debts, dues, extents, executions, judgments, liabilities, obligations,
promises, predicate acts, reckonings, specialties, suits, sums of money,
trespasses and variances, including without limitation court costs and attorneys
fees, in law or in equity, which may exist on the basis of facts other than the
Disclosed Facts.


                                       5
<PAGE>
         The RELEASED MATTERS do not include any claims for the breach by
RELEASEE or any person or entity acting on his behalf of the obligations arising
under this Agreement.

         Nothing in this Release is intended to, or shall be construed or
claimed to be, a license in favor of RELEASEE with respect to any property of
RELEASOR, AVANT! Corporation, or any other person or entity.

         This RELEASE may not be modified or amended except by an instrument in
writing signed by the RELEASOR and the RELEASEE

RELEASOR                                  RELEASEE
SYNOPSYS, INC.
                                          -----------------
By:                                       Date:
        ----------------------------             -----------------

Title:
        ----------------------------

Date:
        ----------------------------

                                       6
<PAGE>
                                    EXHIBIT B

         Sheng-Chun Lo, on behalf of himself and his heirs, executors, agents,
attorneys, administrators, successors and assigns (collectively, "RELEASOR"),
for good and valuable consideration received from RELEASEE (as hereinafter
defined), releases and forever discharges Synopsys, Inc., its corporate parents,
subsidiaries and affiliates, including without limitation Avant! Corporation,
and its present and former directors, managing directors, officers, control
persons, stockholders, employees, agents, attorneys, administrators, successors
and assigns (collectively, "RELEASEE") from any claim or cause of action,
accounts, agreements, bonds, bills, covenants, contracts, controversies, claims,
damages, demands, debts, dues, extents, executions, judgments, liabilities,
obligations, promises, predicate acts, reckonings, specialties, suits, sums of
money, trespasses and variances, including without limitation court costs and
attorneys fees, in law or in equity, which RELEASOR has as of the Effective Date
against RELEASEE that are based on actions or omissions of RELEASEE occurring
prior to the Effective Date which, as of the Effective Date (A) are known to
RELEASOR, (B) have been disclosed in a publicly available report or other
document filed on or before the Effective Date with the U.S. Securities and
Exchange Commission by or on behalf of RELEASEE or the Company, or (C) have been
disclosed to RELEASOR by RELEASEE or the Company on or before the Effective
Date, the information described in (A), (B) and (C) being hereinafter referred
to as the "Disclosed Facts" (collectively, the "RELEASED MATTERS"). For the
purposes of this paragraph a claim against RELEASEE shall be deemed to have
existed as of the Effective Date if such claim is based upon actions or
omissions of RELEASEE that occurred or began prior to the Effective Date, even
if (i) RELEASOR's right to damages or equitable relief on such claim had not
matured as of the Effective Date, or (2) RELEASOR became entitled to additional
or different legal or equitable relief on such claim after Effective Date as a
result of a continuation of the same actions or omissions or the effects
thereof, and/or the passage of time, after the Effective Date, and such claim
shall be deemed to include the right to any such additional or different relief.
THE PARTIES ACKNOWLEDGE THAT THEY ARE FAMILIAR WITH SECTION 1542 OF THE CIVIL
CODE OF THE STATE OF CALIFORNIA, WHICH PROVIDES AS FOLLOWS:

         "A GENERAL RELEASE DOES NOT EXTEND TO CLAIMS WHICH THE CREDITOR DOES
NOT KNOW OR SUSPECT TO EXIST IN HIS FAVOR AT THE TIME OF EXECUTING THE RELEASE,
WHICH IF KNOWN BY HIM MUST HAVE MATERIALLY AFFECTED HIS SETTLEMENT WITH THE
DEBTOR."

         RELEASOR EXPRESSLY WAIVES AND RELINQUISHES ANY RIGHT OR BENEFIT WHICH
IT HAS OR MAY HAVE UNDER SECTION 1542 OF THE CIVIL CODE OF THE STATE OF
CALIFORNIA, OR ANY OTHER STATUTE OR LEGAL PRINCIPLE WITH SIMILAR EFFECT WITH
RESPECT TO CLAIMS BASED ON OR ARISING FROM THE DISCLOSED FACTS, BUT RELEASOR
DOES NOT WAIVE OR RELINQUISH ANY SUCH RIGHT OR BENEFIT WITH RESPECT TO CLAIMS
THAT ARE NOT BASED ON OR DO NOT ARISE FROM THE DISCLOSED FACTS.

         RELEASEE acknowledges that he is aware that, after executing this
RELEASE, RELEASOR or its attorneys or agents may discover facts in addition to
or different from the Disclosed Facts, and that it is the intention of RELEASOR
and RELEASEE not to settle and release any claim or cause of action, accounts,
agreements, bonds, bills, covenants, contracts, controversies, claims, damages,
demands, debts, dues, extents, executions, judgments, liabilities, obligations,
promises, predicate acts, reckonings, specialties, suits, sums of money,
trespasses and variances, including without limitation court costs and attorneys
fees, in law or in equity, which may exist on the basis of facts other than the
Disclosed Facts.


                                       7
<PAGE>
         The RELEASED MATTERS do not include any claims for the breach by
RELEASEE or any person or entity acting on his behalf of the obligations arising
under this Agreement.

         This RELEASE may not be modified or amended except by an instrument in
writing signed by the RELEASOR and the RELEASEE.

RELEASOR                                       RELEASEE
                                               SYNOPSYS, INC.
---------------------------------------
Date:                                          By:
       --------------------------------              ---------------------------
                                               Title:
                                                     ---------------------------
                                               Date:
                                                     ---------------------------


                                       8