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Background

I'm currently working with two quantum software packages: Gaussian 09 and CFour. CFour does a very good job giving explicit control over how you want to invoke frozen core approximations and explains how the default handling is done via the Frozen_Core=On option.

Gaussian 09 has a documentation page regarding frozen core. All my Gaussian 09 jobs with frozen core use the FC flag in the route section. It is important to note that frozen core is basis-set dependent, meaning, the criteria for choosing which core(s) to freeze is directly correlated with the implemented basis set.

I've implemented the following basis sets in Gaussian:

  • aug-cc-pVDZ
  • aug-cc-pVTZ
  • aug-cc-pVQZ
  • aug-cc-pV5Z
  • aug-cc-pV6Z
  • aug-cc-pV(T+d)Z
  • aug-cc-pV(Q+d)Z

What I Need

I need to know if CFour and Gaussian 09 handles the frozen core approximation the same (particularly related to the listed basis sets above). More specifically, if I use the FC option in Gaussian, will invoking the FROZEN_CORE=ON option be sufficient for comparable data or will I need to use the DROPMO command and explicitly tell CFour which core(s) to freeze? If the latter, how do I determine which orbitals to freeze as to match that of Gaussian 09?

The answer may be right in front of me on the documentation pages but it seems rather ambiguous. Of course some benchmark computations could be done to analyze this but I'd rather ask before embarking on such an endeavor that eats precious CPU clock cycles.

EDIT:

This question is strictly about program handling and job control.

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Worth mentioning: A preemptive apology if the answer lies within the given documentation. I will declare that I do not know enough about the topic to figure this out for myself within the given information. Besides, this question could be beneficial to the community as I've scoured the internet looking for the answer but to no avail. –  LordStryker Feb 1 '12 at 23:19
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I strongly recommend that everyone read banned by Gaussian before answering this question. –  Jack Poulson Feb 2 '12 at 0:04
    
Why would this be considered a 'bannable' offense? The documentation is clearly written on both websites. The question at hand is asking for clarification or other references. Timings are not being compared. Performance is not being measured. Code is not being borrowed. I'm confused by your implications. –  LordStryker Feb 2 '12 at 0:08
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I am not claiming that you would be banned, but simply trying to raise awareness for those who might answer that there are potential implications. –  Jack Poulson Feb 2 '12 at 0:16
    
Well for those who may not be familiar to the infamous Gaussian bans, it is important to note that the question posed arises from direct, official, and public documentation of both programs. I look forward to hearing the community responses. –  LordStryker Feb 2 '12 at 0:29
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2 Answers

up vote 1 down vote accepted

The canonical way to freeze cores as described on the Gaussian keyword page is to freeze all non-valence molecular orbitals (MOs) as determined by SCF. This means that for $n$ core electrons the first $n/2$ MOs in ascending MO energy are frozen. This applies only for post-SCF methods. If you want to ensure that CFOUR and Gaussian09 freeze the same MOs you should probably specify explicitly which MOs to freeze as per the ListWindow keyword in Gaussian.

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CPU cycles are not precious. In the time you spent posting this question, you could have run e.g. H2O with MP2/cc-pVDZ in both codes and compared the results, which would answer your question sufficiently.

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You must be new here. –  LordStryker May 8 '13 at 11:49
    
Two months longer than you :-) –  Jeff May 10 '13 at 21:25
    
This post does not add any helpful information explaining how CFOUR explicitly handles the treatment of freezing core or sheds any light on how to manipulate CFOUR to produce desired results. You are simply offering a 'trial-and-error' solution to the question proposed, a poor quality response from such a self-pronounced dedicated member of this community. –  LordStryker May 13 '13 at 19:57
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