Subject: NO SUBJECT

From: <[email-masked]>
Date: Wed, 14 Aug 1996 17:10:00 EDT

In a recent note, it was stated,

>I have been working on a carbene rearrangement scheme and have, after
>much effort, been able to complete all of the calculations at the
>RHF/6-31G** level.  These calculations include both ground and transition
>state structures.  Each of these has been characterized by a frequency
>calculation and found to be a valid point on the potential energy
>surface.  The next step is to do single point calculations using a
>post-HF or correlated method.  This is where I have encountered much
>difficulty.
>
>I have attempted MP2, MP3, MP4, MP2(full), QCIS, QCISD, and QCISDT
>methods, but each has either failed or was beyond the scope of the
>computer resources at my disposal.  By failure I mean that the energy
>values of some of the transition states was below that of the starting
>material...

The poster goes on to ask about using single point DFT at RHF geometries.
However, I think the point of using single point MP2 (or higher) calculations
at RHF geometries needs addressing.  Limited as my knowledge and experience
may be, it seems that the problem is that transition state geometries can be
greatly affected by inclusion of electron correlation, often more so than
is the case for minima on the PES.  Thus, the hoped-for cancellation of errors
implied in using electron correlated energies at non-correlated geometries may
not obtain for transition states as readily as for minima, leading to the
problems mentioned.  If the system size/computer resource combination allows,
wouldn't it make sense to reoptimize all the stable points identified on the
PES at a post-HF level and see if the TS energies thus obtained "behave"
themselves?

Pat