Subject: Problems with the UFF in Gaussian 98

From: Luca Schiaffino <[email-masked]>
Date: Fri, 28 May 1999 16:02:48 +0100

We have experienced some problems when performing geometry optimizations
with the Universal Force Field (UFF) as implemented in Gaussian 98:
single point calculations on minimum energy structures result in
energies very much lower than the final one of the optimization.
Moreover, reoptimization of these structures leads to a very different
geometry, with an energy lower than that of the starting structure.
This strange behaviour is very evident in the case of large molecules or
complexes, and appears to be absent in the case of small molecules.

We suspect that this problem is related to the evaluation of partial
charges and/or electrostatic interactions:

- when the keyword iop(4/67=2) is included in the input file, the
program does not assign partial charges (consequently we obtain a zero
dipole moment even for highly unsymmetrical molecules) and electrostatic
interactions are not evaluated (i.e. they are all zero). In this case we
do not experience any problem and reoptimization of minimum energy
structures leads to the input geometry.

- when the keyword iop(4/67=221) is included, the program assigns
partial charges to all atoms and we experience the problems mentioned
above.

- when we specify the partial charges in the input file (i.e. the
program does not assign partial charges, but evaluates electrostatic
interactions by using the ones we have specified), we do not experience
any problem and again reoptimization of minimum energy structures leads
to the input geometry.

Of course, we observe the same problems even when we use UFF for the
lower layer in ONIOM calculations and for this reason we have not yet
been able to obtain minimum energy structures of large molecules with
the use of ONIOM method.

We would like to know is someone else has experienced the same problems.

Greetings

Dr. Luca Schiaffino

Dipartimento di Chimica
Universita' di Roma "La Sapienza"
e-mail: [email-masked]