Reliable frequency and Raman for some specific states.
Just Got Here
6:06:07 AM PST - Fri, Jan 8th 2016
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.

Forum Regular
5:11:57 PM PST - Sat, Jan 9th 2016
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

Just Got Here
5:23:35 AM PST - Mon, Jan 11th 2016
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.

Forum Regular
3:03:54 PM PST - Wed, Jan 20th 2016
You need to distinguish between a result that is mathematically sound and a result that is physically sound. As long as you are at a stationary point in the nuclear and electron spaces (e.g. a minimum of the potential energy surface and your wave function is a solution of the SCF equations), then you can get a valid solution of the equations for the frequencies, etc. However, that solution may or may not be of any physical relevance (e.g. does a higher energy solution of the SCF equations actually correspond to an excited state of the molecule?).

Just Got Here
8:02:27 PM PST - Wed, Jan 20th 2016
Dear sean,

Thanks for your comment.

The higher energy state is supposed to act as a control group, providing a comparison in particular cases. Its mathematical meaning instead of actually physical background appropriately matters in this case for understanding some questions we concerned about. It may be sort of similar to those CDFT results in understanding the dependence of some molecular properties on charge or spin distribution where a curve is often plotted though not each data point corresponds to a practically existing state and can also be taken as offering a mathematically control group.

Is it possible to obtain the freq or Raman solely corresponds to a certain higher energy state? And if yes, is there some particular settings should I use or take care of?

Forum Regular
10:56:05 AM PST - Thu, Jan 21st 2016
It is difficult to provide a general answer to that question, and I suggest you spend sometime searching the literature for background and other examples of the method you plan on using. If you are breaking spin symmetry (i.e. an unrestricted wave function) you are inevitably going to get mixing of higher spin states into your wave function (this is going to be true of the lowest energy state as well as all higher energy states whether calculated through an SCF procedure or TDDFT). I am not very familiar with CDFT, so I cannot provide any insight there; although, I am unsure if the constraints are properly accounted for in all of the modules beyond the SCF module and the gradient module (for charge constraints). You would still be able to get frequencies through a numerical procedure, but if the constraints aren't accounted for in the CPHF routines, I would not expect a Raman calculation to work/provide meaningful results.

Just Got Here
7:52:28 PM PST - Thu, Jan 21st 2016
Thanks for all the help. I'll follow the suggestions.


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