Subject: CCL:SINGLE-POINT DFT ON HF GEOMS

From: [email-masked] (Alan Shusterman)
Date: 14 Aug 96 15:17:42 PDT

Errol Lewars wrote:
SP MP2 energies on HF/6-31G* geoms are often done, but I don't trust them (does
any one have a compilation of these calcs, comparing them with exp. or
higher-level calcs?): the geom can change too much going from the HF to the MP2
level.

Response:
A small compilation can be found in Practical Strategies for Electronic
Structure Calculations, W.J. Hehre, Wavefunction, 1995.

The upshot is that transition state geometries vary much more with the level of
theory than do equilibrium geometries, but energy barriers need not.  Although
it is easiest to think about cancellation of errors resulting from "parallel"
energy surfaces, one need not have a surface that parallels the true surface to
get cancellation of errors and a reasonable energy barrier.

Two examples drawn from Hehre's book (p 106-107), rearrangement of
methylisocyanide to acetonitrile, and elimination of formic acid from ethyl
formate, illustrate what may (?) be typical results.

Single point DFT barriers calculated using either AM1, HF/3-21G, HF/6-31G*,
MP2/6-31G* geometries all agreed to within 3 kcal for the first reaction and to
within 7 kcal for the second (DFT//AM1 results were usually the outliers;
excellent agreement was seen between DFT//HF and DFT//MP2 barriers).

Single point MP2 barriers calculated using the same geometries used above were
in even better agreement, i.e., choice of geometry mattered less.

The differences in DFT and MP2 barriers for a given reaction and given method
for obtaining geometries were larger than the differences caused by changes in
geometry.

In short, several energy surfaces that have their stationary points in the
wrong places and which cannot, therefore, be parallel to the true surface or to
each other, may still provide similar estimates of energy barriers.

Alan Shusterman
Department of Chemistry
Reed College
Portland, OR  97202