Subject: Initial Hessians

From: <[email-masked]>
Date: Tue, 31 May 94 08:17:10 -0400

Netters: 

The question has been raised as to the best initial Hessian for ab initio
calculations.  Someone commented "what could be better than the analytical
ab initio hessian at the starting geometry?"  Others suggest from
semiempirical methods.  I have another point of view.  What I have found to
be the most useful, assuming that a ground state is sought, is to be sure
that a reasonable starting geometry with a reliably positive definite
hessian is obtained.  I have found that the source of the hessian is not as
critical when there are no critical, low energy torsions.  Things are quite
different when there are low energy torsions.  The program I use almost
exclusively for geometries is GAUSSIAN-92, and I have determined a rather
robust strategy for starting off an ab initio run. 

First the initial geometry is obtained and optimised in GAUSSIAN using PM3.
Next the ab initio run is started using OPT=MNDOFC iop(4/20=7).  (For the
iop overrides the comments in fortran source for the 400 links must be
consulted for all the details.  The provided documentation leaves some of
this out.) The iop overide is the critical issue here... By default you
will get an AM1 hessian, regardless of whether or not AM1 or PM3 is
initially used for an initial geometry.... It is vital that the initial
geometry and the initial hessian be determined BY THE SAME METHOD. This is
true, regardless of the method used! (Exceptions can certainly be found
when high level basis sets are used in ab initio determinations.)   For
example, when torsion angles differ by 20-30 degrees between PM3 and AM1,
and the initial geometry comes from PM3 (RPM3) and OPT=MNDOFC alone is
used, the AM1 hessian can lead to major grief. 

I have found that for floppy molecules this approach works extremely well,
and thus should work for all molecules....  It assumed that you are using
the semi-empirical method or basis set of your choice. 

John McKelvey 
Eastman Kodak 
[email-masked]