Subject: CI-Singles - Wrong Excited State Energies?

From: Demetrio Antonio da Silva Filho <[email-masked]>
Date: Tue, 11 Dec 2001 16:19:08 -0700

Dear CCL'ers,

I am using CI-Singles (in gaussian'98) to optimize the geometry of the first
singlet and triplet excited state.

After getting the energies of those states, I subtracted those numbers from the
ground state energy and then compared with the excited state energies obtained
by running a CI-S calculation  using the ground state geometry.

To my surprise, I found that the first excited state energy, obtained using 
the ground
state geometry, is SMALLER than the energy difference between the first 
excited state
and the ground state energies.

It means that when I optimize the geometry of the excited state using 
CI-Singles, I arrive
at a geometry which energy is HIGHER than the one obtained using the ground 
state geometry
-- the one that I used to begin the CI-S calculation.

Does anybody have any idea of why I get these strange results? Those 
results mean that CI-Singles is not a reliable methodology to optimize 
excited state geometries?

Thanks in advance,

Demetrio Filho