## VMD-L Mailing List

**From:** Mayne, Christopher G (*cmayne2_at_illinois.edu*)

**Date:** Thu Aug 20 2015 - 13:09:10 CDT

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Francesco,

There are two places in the workflow where substituting MP2 geometry optimization with SCP opt + MP2 single point would matter: determining a low energy conformation of the molecule, and as input for the Hessian calculation, which I'll deal with in order. Technically speaking, the ffTK parser should handle the change in theory without a problem. You can check this by loading the optimization log and checking that VMD reads in multiple conformations into the OpenGL window. From a theoretical perspective, as long as the resulting geometry is of sufficient energy it should suffice for use in subsequent steps in the workflow, e.g., water interaction profiles. Whether this works with the Hessian calculation is less clear to me a priori, and may work for some cases but not others. The input file for the Hessian calculations tells Gaussian to read the initial coordinates from the checkpoint file output during the geometry optimization. I could imagine that an SCP-computed geometry minimum may differ from MP2 such that you would end up with problems in the frequencies. I would have to run test cases to say much more. Alternatively, if you think you have sufficient justification, you could perform all calculations as the SCP level of theory, and the ffTK parsers should continue to work (fingers crossed!).

Regards,

Christopher Mayne

On Aug 20, 2015, at 12:50 PM, Francesco Pietra wrote:

Hi Chris:

Sorry for having missed your answer. I also came across those suggestions by Gaussian, but that was not the reason. As I said it was lack of sufficient memory.

I still have to become comfortable with "divide and conquer". As it is implied in your answer that it is of such accuracy as to demand OPT/MP2, I'll try to learn how to do.

At any event, would ffTK accept the Gaussian log file from geometry optimization at SCF level, followed by single-point MP2? Or is ffTK expecting a log for geometry optimization at MP2 level?

Thanks a lot

francesco

On Thu, Aug 20, 2015 at 7:23 PM, Mayne, Christopher G <cmayne2_at_illinois.edu<mailto:cmayne2_at_illinois.edu>> wrote:

Francesco,

I responded here: http://www.ks.uiuc.edu/Research/vmd/mailing_list/vmd-l/26205.html

On Aug 20, 2015, at 10:13 AM, Francesco Pietra wrote:

Thanks a lot for advice on strategy.

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