Subject: CI-S geometry optimizations

From: Demetrio Antonio da Silva Filho <[email-masked]>
Date: Wed, 02 May 2001 23:46:57 -0700

Dear CCL'ers,

I want to optimize the geometry of the ground state, the first excited triplet
state and the first excited singlet state of a molecule.

I tried to do that using G98 and the CI-S methodology. I specified
the following keywords:

#p opt  cis=(direct,Root=0)/3-21G*               -> for ground state (GS).
#p opt  cis=(direct,Root=1,Singlets)/3-21G*   -> for first excited singlet 
state (S1).
#p opt  cis=(direct,Root=1,Triplets)/3-21G*   -> for first excited triplet 
state (T1).

Is that right?

I am confused because as result I got : E(GS) = E(S1) > E(T1) and it is 
completely different
that what I expected (i.e E(T1) > E(S1) > E(GS) ).

I don't know if I can expect that CI-S will give me the right order of 
those states. Does anybody
have experience with this problem?

As I have the geometries for those states, I can do a single point 
calculation and try to get a
reasonable energy. Does anybody have a suggestion on which methodology I 
should use?

Thanks in advance,

Demetrio Filho