Subject: Re: Symmetry

From: [email-masked] (Wilson_Bruce)
Date: Tue, 26 Apr 1994 15:20:06 -0400

Regarding the use of symmetry in quantum calculations.  Most of the
stuff I've looked at never was more than C1 to start with so it's not
been a problem.  By tinkering with the Z-matrix and enforcing certain
types of symmetry (like requiring selected C-H bonds to have the same
length), I've seen between 10 and 300% differences in speed for test
systems I've worked with.  For example, in dimethyl terephthalate, I
can require all of the aromatic C-H bonds to be of the same length,
and I can require certain other bonds (e.g. both carbonyls) to be the
same length.  Any time I'm really interested in the most accurate,
quantitative results, I'll do a symmetry disallowed optimization, or
at least a symmetry disallowed frequency calculation, to see how
far from a true minimum I happen to be.

My opinion (worth what you paid for it ;-) ):  Symmetry can be used
to advantage, particularly less restrictive symmetry than full point
group symmetry, but you take a risk of getting a misleading answer
when you do that.  The classic one (and one that a student of mine
got tripped up on this fall) was keeping the symmetry and getting
a planar biphenyl.  If I'm really concerned, I've been known to run
the starting geometry through a few femtoseconds of dynamics to 
completely ruin any symmetry/stationary points I've happened to 
introduce in constructing the molecule.

		Bruce Wilson ([email-masked])