| 6:26:53 PM PDT - Mon, Aug 19th 2013 |
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YOU WILL GET WRONG RESULTS
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The statement "YOU WILL GET WRONG RESULTS" is meant to warn the user as to the unverified nature of the CCSD imaginary response code. I tried very hard to make it work and rewrote the code half a dozen times but kept getting inconsistencies w.r.t. previous ones. There is no reference work to which I can compare directly.
It is entirely possible that the code is correct but I won't certify that and you should not publish results claiming you've done CCSD C6 in NWChem unless you have an exact numerical verification against an independent implementation.
That the C6 code is undocumented in the user manual should indicate that it is not to be used. The "YOU WILL GET WRONG RESULTS" warning further amplifies that point.
If you want C6 coefficients, I recommend you use the Cauchy moment method based upon the polarizability for real frequencies. It is documented in the literature but not implemented in NWChem. You can write Python or Matlab or ... though since it is not compute-intensive to solve these equations.
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