Subject: transition state problem (?)

From: "Joseph W. Bausch" <[email-masked]>
Date: Sat, 21 Mar 1998 12:25:57 -0500

To the CCL community:

	I am interested in locating a transition state and determining the
energetics of a degenerate process which I'll represent as A --> TS --> A'.
Using G94, I employed a QST2 calc at HF/STO-3G using A and A' HF/STO-3G
geometries (with atoms ordered properly) as input.  The method appeared to
work fine as the transition state it found looked like what I "expected",
and the relative energy is about 7 kcal/mol higher than A.  I then took
this geometry and did an OPT=TS calc at HF/3-21G and this job ran
successfully, with the relative energy being about 5 kcal higher than A.
To confirm that this transition state connected A and A', I ran an IRC calc
at 321g, and this worked.
	So, desiring to have higher level of theory data, I then did an
OPT=TS calc at HF/6-31G*, which ran successfully, this time the TS being ~4
kcal/mol higher in energy than A.  The results of the calculations I'll
describe next have me a bit confused.
	I have run OPT=TS calculations at B3LYP/6-311G* and also at
MP2/6-31G*,  and both of these calculations ran to completion.  However,
the energy of TS at B3LYP/6-311G* is ~2 kcal/mol MORE stable than A, and at
MP2/6-31G* TS is MORE stable than A by ~11 kcal/mol!  Has anybody out there
had a similar situation to this?  Can I assume that DFT and MP2 levels just
aren't very good for the energetics of transition states, or are these
results perhaps peculiar to the specific molecule I am investigating?  If
my DFT and MP2 levels of theory aren't good for this type of problem, what
level of theory is considered reliable for this type of calculation?

Cheers and TIA,
Joe
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