From NWChem
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Just Got Here
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6:40:36 AM PDT  Thu, Jul 20th 2017 

Dear all,
I have carried out a vscf calculation and I could find the VSCF anharmonic frequencies in the output. However, I am also looking for the cubic and quatric anharmonicities and I could not find them in the output or the ecce.out file.
Does anyone know if and how those can be extracted?
There are however several output files like the *.coup file but it is not clear to me what the numbers in this file actually represent.
Best wishes
Nicole




Gets Around
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4:28:11 AM PDT  Mon, Jul 24th 2017 

VSCF doesn't have cubic and quartic anharmonicities because you are not expanding the nuclear potential in a power series. For the implementation in NWChem, you have the option to calculate the effective potential for the vibrational wave functions as a sum of contributions from individual modes, pairs of modes, or triplets of modes, with the code calculating a 1, 2, or 3dimensional grid of the energy for displacements of the geometry along each relevant mode. The total vibrational wave function (approximated as a product of single mode wave functions) is then calculated using this effective potential. VSCF is a bit like HartreeFock for the nuclear vibrations.
The *.diag file contains the 1dimensional grid for each mode. The *.coup file it appears was intended to contain the 2dimensional grid for each pair of modes, but it seems it overwrites itself all the time so only the last result for the last pair of modes ends up in the file at the end of the calculation. For the 3dimensional grid for triplets of modes, it looks like the grid would get written to the normal output file, but I'm not sure because I've never run one of those calculations since they are very expensive for anything but very small systems.




Just Got Here
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5:39:19 AM PDT  Mon, Jul 24th 2017 

Thanks a lot for the explanation, Sean!
That was very helpful to me.
Best wishes,
Nicole



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