Subject: SINGLE-POINT DFT ON HF GEOMS

From: "E. Lewars" <[email-masked]>
Date: Wed, 14 Aug 1996 17:14:12 -0400 (EDT)

Hello,  Jim Pugh asked if single=point DFT calcs on structures optimized 
at the HF/(6-31G*?) level were valid.
  The short answer is that if gives the right answers it's valid: one
could assemble some experimental results then compare them with relative
energies calculated this way.
  the long answer is this: Single-point MP4 and QCISD(T) on MP2/6-31G*-opt.
structures are "valid" (often give reasonable answers, I think).  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.  Thus while the MP4 and QCI pot. E surfaces are (we
hope) approx. parallel to the MP2 PES, the MP2 PES may not be parallel to
the HF surface.  Roughly parallel PES's are needed to avoid the absurdity
of a TS being lower in E than one of the two points it is supposed to 
connect (I found this absurdity with MP2 SP calcs on HF geom's).
  There is a paper by Schleyer et al, ca. 1991, "The inportance of MP2
optimizations"--- I could locate it.
Errol Lewars
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