Subject: SUMMARY:CASSCF: StateAverage calculation

From: "Dr. Hatice Can" <[email-masked]>
Date: Thu, 29 Aug 2002 18:33:30 +0200

Dear All,

I am summarising the responses those were sent to me for my following
question.The question:

I am doing CASSCF calculations on a cation. I am not
clear about why state average calculations is done. What for is it? Is it
really necessary? If you would sent any suggestions or literature name, I
will be glad.

Responses:

Tomasz Borowski <[email-masked]
Dear Hatice,
         State Averaged CASSCF calculations are usually done to get
         the wave functions and enegies of higher excited states. For
         example, it is normally possible to converge the single root
         calculations for the second or third state for given symmetry.
         However, it is almost always impossible to get higher roots
converged
         (due to root floping or other numerical problems). Thus, if you
are
         interested in, let say, five lowest states of a given symmetry
and
         you want to use CASSCF, then you have to resolve to state
averaged
         calculations, unless you are exceptionally lucky :)
         Kind regards,
         Tomek

[email-masked]

Hi Hatice,
The CASSCF like an ordinary SCF calculation is an iterative
process,
         minimizing the energy until its change is within certain cutoffs.
If yo u are doing calculations on say the ground and one excited state you
         can choose to minimize the AVERAGE of the energies of the two
         states, this is known as state averaging. An alternative would be
to
         minimize the energy of the ground state, but this might be seen
as
         being "biased" towards the ground state.
         Hope this helps,
         Cheers, Tim