Subject: correlated SP calcs on HF geoms...

From: <[email-masked]>
Date: Thu, 15 Aug 1996 9:12:03 -0500 (EST)

Alan Shusterman writes:

> ...
> 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.
> ...

Just to comment a bit further about my experience in this area. We looked
at the ethylformate --> ethylene + formic acid TS (See JACS, 1994, 116, 6271)
at both the MP2/6-31G*//MP2/6-31G* and RHF/6-31G*//RHF/6-31G* levels. Although there
were some differences in the TS geometries, the differences were not enough
to change any conclusions in the study.  In fact the Eact in the
MP2/6-31G*//MP2/6-31G* and in the MP2/6-31G*//RHF/6-31G* single point
calculation were almost identical. This work, however, does hilight the
influence electron correlation can have on TS geometries. So care should be
used in substituting single point calculations with correlated methods for
full optimization without examining at least one case optimized at both levels.

Jon Erickson
DowElanco CAMD
[email-masked]