From chemistry-request@ccl.net Sun Apr 19 12:30:30 1992
Date: Sun, 19 Apr 92 09:17:07 EDT
From: m10!frisch@uunet.UU.NET (Michael Frisch)
Subject: Semiempirical optimizations in MOPAC
To: chemistry@ccl.net
Status: R


There seems to be some confusion about what Gaussian does during
semi-empirical optimizations.  Here are some details:

1.  Gaussian defaults to accuracy comparable to that produced by the
    PRECISE keyword in MOPAC.

2.  The step-size used in numerically differentiating the integrals as
    part of the gradient calculation is small in Gaussian, because a
    smaller step can be taken reliably if it is known in advance that
    PRECISE is always turned on.  This provides somewhat greater
    accuracy in the derivatives.  I think something like this (but
    with different values for the step size) is done in MOPAC version
    6 but not in earlier versions.

3.  Only the energy and cartesian gradient are computed using code from
    MOPAC; the rest of the optimization uses the standard Gaussian
    routines in exactly the same manner as an ab initio optimization
    would.

4.  The cartesian forces are converted to internal coordinates using
    different algorithms.  I don't know the details of how MOPAC does
    the conversion, but this appears to make a difference.  Gaussian
    uses analytic expressions and doesn't suffer from the sensitivity
    to internal coordinate definition that MOPAC apparently does,
    since MOPAC gives the warning message about small changes in
    internal coordinates causing large displacements for cases where
    Gaussian does not have difficulties.

5.  Gaussian's default optimizer is not the same as either EF in
    Gaussian or EF in MOPAC.  It does use an RFO (Rational Function
    Optimization) step for the quadratic portion of the step as in the
    EF method, but it also does gradient-based linear corrections at
    every step as suggested by Schlegel, and it uses an improved
    version of Schlegel's update scheme for the Hessian.  EF (Baker's
    algorithm) is available as an option, but is usually inferior to
    the default, since the default is a mixture of the best parts of
    Baker's and Schlegel's methods.

Michael Frisch
Gaussian, Inc.
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