Subject: what is wrong?

From: <[email-masked]>
Date: Tue, 8 Mar 94 10:48 CST

Dear Netters:

      I am having some difficulties to explain my calculation results.  I have
carried out some calculations for a redical molecule by using Gaussian-92.  I 
first using UHF-MP2 with 6-31G* bases set to optimize the molecule's geometry.
then I started the frequency calculation.  It is common to get the geometry 
optimization using high lever calculation (like MP2 and MCSCF), but frequency
calculation using "low lever" calculation (same or even smaller basis set like
6-31G* without MP2, even 3-21G).  So I did both calculations.  First I used same
lever (UHF-MP2-6-31G*) to compute the normal mode frequency, then I changed to
UHF-6-31G* without MP2 to do the same frequency calculation using the same 
geometry (obtained by UHF-MP2-6-31G*).  My results showed that the two set of
calculation gives quite different result (for some important mode assignment).
This means that the electron correlation affects the normal mode calculation.
Because the wavefunctions for the two set of calculation are different (with
and without electron correction).  Am I right?  If this is the case, how can I
extract the coefficient of the Slater determinant from the MP2 calculation.
I really wanted to know how many electronic states involve in my calculation,
and how much of the each state.  I appreciates your help and your 
attention.  Any comments, suggestions or references will be a great help.
Thank you all in advances.  My e-mail address is [email-masked]

                                               yueyi  qin

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From: Robert De Niro <[email-masked]>
Date: Tue, 8 Mar 94 19:28:28 "GMT

Mr (or Dr.) Qin,

The problem here (which has been brought up before) is that is Gaussian-92,
analytic derivatives and 2nd derivatives are available for MP2 calculations.

This means that an HF frequency calculation and an MP2 frequency calculation
are entirely different. Previous versions of Gaussian could only do analytic
2nd derivs. for HF, so HF frequencies were frequently reported to avoid time
consuming numerical methods. I am not sure of the validity of an HF frequency
job at an MP2 geometry, because the geometry would not be a stationary point
at the lower level, so the normal modes calculated would be invalid anyway.

Craig Wilson, Univ. of Warwick, England, UK.