From NWChem
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2:32:06 PM PST  Fri, Dec 9th 2016 

Dear All,
I am doint dft calculation of a system with degenerate HOMOLUMO, They are supposed to be degenerate, because of symmetry of ionic structure. I am aware that enforcing symmetry by "autosym" may cause some problems in evaluating matrix element. So, I tried two approaches to fix this issue, but they end up with different results. So, I would like to some help to have correct calculations.
The first approach is not to use symmetry by 'noautosym'. Calculations go without warings, but in the end result HOMOLUMO are no longer degenerate, which is a concern for me.
The second approach is to use open shell wave function by 'odft' and symmetry by 'autosym'. Calculations come with waring "Warning  the HOMO and LUMO are degenerate and you are using symmetry..." and "Warning: spatial symmetry breaking in UKS" In the end results, HOMO LUMO are degenerate, which is good. But the total energy is different from nonsymmetry calculations, as described in the first approach.
So, which one should I trust? I guess nonsymmetric calculations are usually more reliable, but since HOMOLUMO are nondegenerate in my case, I don't know what to believe any more... Thank you for your help.
Cheers
Jia




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3:19:23 PM PST  Fri, Dec 9th 2016 

Jia,
I'm not sure what system you are studying, but in my work with transition metal complexes, the HOMOLUMO degeneracy is usually an indication that symmetry will be broken by a JahnTeller distortion. In this case, it is better to "break" the symmetry to a lower point group and allow the HOMO and LUMO to split in energy. This doesn't necessarily mean disabling symmetry outright  a JahnTeller distortion can leave the molecule in a subgroup of the original point group provided that the HOMOLUMO degeneracy is broken.
Best,
James




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3:50:08 PM PST  Fri, Dec 9th 2016 

Dear James,
I am aware that the system is unstable with respect to JahnTeller distortion. But it is still a well defined question to ask, what is the total energy of an unstable structure. Actually, I am trying to parameterize a JahnTeller problem via electronic strcuture calculations.



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