Are NAO energies the same when starting from same orbitals ?

Are NAO energies the same when starting from same orbitals ?

Postby sam84eth » Fri Jun 29, 2018 2:24 am

Hi,

I would like to ask if the NAO orbital energies are always the same, when starting from the same orbitals obtained with Gaussian ?

https://www.chem.wisc.edu/~nbo5/web_nbo.HTM

states that the NAOs are uniquely defined, but on the other hand the expansion into a finite basis set is maybe not unique.

When starting from the same orbitals from a Gaussian calculation, once with NBO 3.1 and once with NBO 5.9 or 6.0, would you expect the same orbital energies for NBOs or can they differ due to numerical noise in the algorithm ? Would a difference in the NBO orbital energies point to a potential bug in NBO ?

Best regards

Sam
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Re: Are NAO energies the same when starting from same orbita

Postby ericg » Mon Jul 16, 2018 7:38 pm

NAOs, and their occupancies and energies, may differ somewhat if comparing NPA results for NBO 3.1, 5.x, and 6.0.

NAOs from 5.x and 6.0 may differ very slightly because NBO6 neglects core-type functions in the final diagonalization step of the NAO algorithm. Including these core functions in the diagonalization, as in NBO 3.1 and 5.x, allowed core NAOs to gain some Rydberg character.

Differences between 3.1 and 6.0 may be significant, likely arising from the expanded valence shells that are the default for some atoms in NBO 3.1 that Gaussian maintains and distributes. For example, NBO 3.1 treats 4p orbitals (in addition to the 4s and 3d) as valence functions for Fe, whereas NBO 6 does not (i.e. 4s and 3d only). The NAO procedure acts to maximize occupancies of core and valence orbitals. Expanding the valence allows the procedure to distribute the electrons across a larger number of orbitals, leading to differing NAO occupancies and character.

Expanded valence shells are not recommended by the NBO team. When reporting NBO 3.1 results, you should acknowledge that valence shells may be non-standard.

Eric
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