From NWChem
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| 5:06:07 AM PST - Fri, Jan 8th 2016 |
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Hi,
I'm a greenhand in using NWChem.
I have used other programs before and learned that the frequency calculation in some packages is not reliable for some states, such as those are neither the ground state under the a specific multiplicity nor excited states which can be handled by TD-DFT methord. It's said the problem origins from the perturbation algorithm, where the lower energy wavefunction could be seen and thus introduce their influence into the results.
In NWChem, there are technics such as CDFT which can obtain some states as described above. Or say, an closed-shell state given the ground state is a broken symmetry state. I wonder can I get reliable frequency and Raman for these states in NWChem?
Thanks for you help.
Best regards.
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Niri Forum:Admin, Forum:Mod, NWChemDeveloper, bureaucrat, sysop
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Gets Around
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| 4:11:57 PM PST - Sat, Jan 9th 2016 |
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Hi, I am not sure if I fully understand your question.
Try using our property module which will allow you to calculate frequencies and Raman spectra.
Best,
-Niri
niri.govind@pnnl.gov
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Just Got Here
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| 4:23:35 AM PST - Mon, Jan 11th 2016 |
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Dear Niri,
Thanks for your suggestion. I will try the using the module.
Sorry, I am not sure about the details in the question myself and, beg your patience, I will try making it clearer below.
It' said the perturbation algorithm used in frequency calculation such as coupled perturbed Hartree–Fock (CPHF) requires the wavefunction of the system to be the ground state wavefunction under its multiplicity. Otherwise, the lower-energy wavefunction would be seen in the perturbation process and make the obtained frequency not only contain the result of the studied higher-energy state. A stability check for the wavefunction is always suggested before such calculations.
This algorithm seems to be adopted for analytic frequency calculation in some cases. For numerical frequency calculation, the same problem is said may also produce because the change of the structure may induce the wavefunction converge to a lower-energy state. Calculation using TD scheme doesn't have this problem.
But if the states is as described in may last letter that can not be obtained from TD and is obviously not the ground state under a certain multiplicity, can I get reliable frequency results for them in NWChem? The calculation can always run though, I think.
Thanks again for your kind help.
Best regards.
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Edited On 4:27:30 AM PST - Mon, Jan 11th 2016 by Jiangwr14
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