Subject: Re: Gaussian: recalculating thermochem

From: Jeremy Greenwood <[email-masked]>
Date: Wed, 1 Nov 2000 14:21:16 +0100

QUESTION

> Gaussian allows the rapid recalculation of thermochemical data   
> (e.g. with new scaling factor, temp., pressure) from the checkpoint
> file after a frequency job using freq=readfc                           
  
> However, if the checkpoint file is missing, it gets more tricky.                
> We would like to recalculate the thermochemistry without having
> to resort to re-running the frequency job.                   
  
> A checkpoint file can be created by a single point calculation.
> The force constants are listed in the archive, and can be read in
> using opt=fccards, or by editing the formatted checkpoint file by
> hand and unformatting, but this doesn't seem sufficient to
> convince Gaussian to perform freq=readfc.       
  
> One could use the information in the output file to recalculate
> the thermochemistry from first principles, but this is a little
> laborious. It would be nice to have an automated procedure.
  
> Does someone have a good way to do this?  

ANSWER using Gaussian, for the cost of a cheap frequency job and
a single point calculation:

1) Run a frequency job at HF/sto-3g using the geometry optimised
at the higher level
2) Format the checkpoint file using formchk
3) Replace the force constants in the checkpoint file with those
>from the archive of the output file. 
Handy extraction script from Per-Ola Norrby:
ftp://compchem.dfh.dk:/pub/QueueScripts/po_getForce
4) Unformat the new checkpoint file with unfchk
5) Run a single point calculation using the checkpoint file, 
specifying geom=check guess=read, and maybe scf=sleazy
6) Run a freq=(readisotopes,readfc) job to recalculate the thermochem

(Thanks to Joseph Ochterski, [email-masked] for help with this)

ALTERNATIVE at virtually no cost

Frank Jensen ([email-masked]) kindly supplied a short Fortan program to 
directly calculate ZPE, and vibrational contributions to H and S from 
a list of the frequencies, temperature and scale factor; from these
the desired quantities can be found.

Cheers,

Jeremy
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Jeremy Greenwood                                 [email-masked]
Department of Medicinal Chemistry                      ph +45 35306117
Royal Danish School of Pharmacy                        fx +45 35306040
Universitetsparken 2, DK-2100 Copenhagen, Denmark
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 I am busy just now again on electro-magnetism, and think I have got  
 hold of a good thing, but can't say. It may be a weed instead of a   
fish that, after all my labor, I may at last pull up. -Michael Faraday
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