Subject: RE: G:Help need with NBO

From: "Stefan Fau" <[email-masked]>
Date: Tue, 16 Feb 1999 13:34:26 +0100

Dear Maija,

the warning about 'Low Occupancy Core Orbitals' is nothing more than a 
warning with cautious thresholds: just have a look at the occupancies 
to confirm that they are close to 2 electrons.

I also found the 'Population Inversion' warning in some of my outputs 
and just ignore it if the inversion is between the occupancies of 
lowly occupied orbitals (that is lower than maybe .02 electrons). 
The warning is more common with extended basis sets because then there 
are more unoccupied basis functions and consequently more NAOs with 
near zero occupancies.

The warning about 'Population Inversion' needs a little background:
The Natural Population Analysis (NPA) is conducted in the basis of 
Natural Atomic Orbitals (NAOs). They are created by orthogonalizing 
the (atom centered) basis set used in GAUSSIAN under the constraint of 
changing highly populated basis functions as little as possible. On the 
other hand, lowly occupied basis functions may change much more. During 
this process the orbitals of the 'Natural Minimal Basis' (consisting of 
the orbital types that are occupied in the ground state of the atoms, 
e.g. 1s,2s and _all_ 2p orbitals in the case of boron) and the orbitals 
of the 'Natural Rhydberg Basis' (the rest) are treated differently. 
(This has led to some discussion on NPA and transition metals, because 
their np orbitals are treated as Rhydberg Orbitals by the NPA although 
they are sometimes populated, e.g. in complexes of late transition 
metals.) I think that the test for 'Population Inversion' checks if 
the NAO occupancies are decreasing with increasing NAO energy. 

Stefan
______________________________________________________________________
Dr. Stefan Fau
Fachbereich Chemie, AK Frenking
Philipps-Universität Marburg
35032 Marburg, Germany
[email-masked]

Edited original message:
> Dear CCLers,
> 
>   I am trying to do NBO (Natural Bond Orbital) analysis within 
> Gaussian98 ( options for calculation b3lyp/6-311+g* scf=tight pop=nboread
> IOP(3/59=9)). In the middle of calculations following warning appear:
>  
> WARNING:  1 low occupancy (<1.9990e) core orbital  found on  P 3
> 
>  WARNING:  Population inversion found on atom  P 3
> 
> I am not sure I understand what happend. Can somebody 
> give me some hints or comments on that?
> Thank you for your help in advance
> 
> Sincerely Yours,
> Maija Lahtela-Kakkonen