Subject: Re: CCL:G:wave function stability question

From: Vitaly Rassolov <[email-masked]>
Date: Tue, 10 Nov 1998 08:39:42 -0600 (CST)

On Mon, 9 Nov 1998, Dale A. Braden wrote:

> Dear CCL,
> 
> I'm doing DFT calculations on an open-shell transition metal compound
> using G94.  I saw that 3 of the virtual orbitals had negative eigenvalues:
>  .....
>  .....
> 
> 1) Am I correct that the stability calculation is NOT guaranteed to
> recognize an excited state?

Yes. Stability calculation checks if you are in a minimum (stable), or a
saddle point (unstable).  It can't tell you if you are in the global or a
local minimum.

> 2) Are negative eigenvalues for virtual orbitals a SURE sign that an
> excited state has been obtained?

No.  Think of a hydrogen in a complete (very large) basis set.  All
(almost all) bound states will have negative eigenvalues.
 
> 3) In case I really still do have an excited state wave function, is the
> only way to direct it towards the ground state to guess which orbitals
> should be occupied, and then use guess=(read,alter) to fix their
> occupations?  (I'd like to avoid hit-or-miss strategies!)

Yes.  There is no sure procedure that moves you towards the global
minimum.  But in practice stability calculation does a very good job.  I
don't know a single case when stable to all perturbations wave function
would still be not in the ground state.

_______

Vitaly Rassolov                      [email-masked]
Chemistry Department                 tel. (847) 491-3423
Northwestern University              fax  (847) 491-7713