[Pw_forum] Projection of dos on to molecular orbitals

Guido Fratesi fratesi at mater.unimib.it
Thu Jun 18 15:05:45 CEST 2015

Dear Vasudevan,

On 18/06/2015 05:29, vasudevan m.v wrote:
> Thanks a lot for the clarification. So if I have 28 eigenstates in full
> system (molecule+surface) and 14 eigenstates (for gas phase molecule in
> same unitcell as that of full system), I can project the whole 28 states
> onto 14 molecular states and then looking at HOMO of molecule,
> integrating it's DoS up to Fermi will provide the population of that
> level in full system. This is what I am supposed to do. I hope I am
> clear and understood correctly.

you got it correctly.
Just beware that, while the assignment of DOS features to molecular 
orbitals is generally unambiguous, the population of the level may be 
affected by incompleteness in the atomic basis set used for the 
projection (and of course arbitrariness of the definition of what the 
HOMO is). Indeed, when we pass through this atomic-basis-set 
representation for the projection we do miss some of the states both in 
the "full system" and "the part" (*). So the quality of the result 
should be analyzed case-by-case.

(just in case: if you plan to integrate the DOS you should use the same 
smearing for plotting as in the SCF calculation)

(*) ideally one would project the states with their PW representation, 
but this has other drawbacks.

Best wishes,

Guido Fratesi

Dipartimento di Fisica
Universita` degli Studi di Milano
Via Celoria 16, 20133 Milano, Italy

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