degenerate HOMO-LUMO and symmetry
Just Got Here
2:32:06 PM PST - Fri, Dec 9th 2016
Dear All,

I am doint dft calculation of a system with degenerate HOMO-LUMO, 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 HOMO-LUMO 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 non-symmetry calculations, as described in the first approach.

So, which one should I trust? I guess non-symmetric calculations are usually more reliable, but since HOMO-LUMO are non-degenerate in my case, I don't know what to believe any more... Thank you for your help.

Cheers
Jia

Clicked A Few Times
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 HOMO-LUMO degeneracy is usually an indication that symmetry will be broken by a Jahn-Teller 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 Jahn-Teller distortion can leave the molecule in a subgroup of the original point group provided that the HOMO-LUMO degeneracy is broken.

Best,
James

Just Got Here
3:50:08 PM PST - Fri, Dec 9th 2016
Dear James,

I am aware that the system is unstable with respect to Jahn-Teller 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 Jahn-Teller problem via electronic strcuture calculations.


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