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At least some of your answers are covered by an excellent writeup at:

http://www.scm.com/Doc/Doc2002/Theory.pdf

and the references given in that document.

Serguei+

On Thu, 23 Jan 2003, Michael Drummond wrote:

>
> I have a variety of related questions about how one goes about
> comparing
> the energies (Total Bonding Energies) for molecules in various
> non-ground
> states.
>
> (1) If I optimize the geometry of a molecule in its ground singlet
> state
> and then in its lowest triplet state, how do I calculate the energy
> difference between these two states?
>
> (2) What about comparing the energies between a molecule optimized in
> its
> ground singlet state and then in its first excited singlet state?
>
> (3) How is TD-DFT best used to address issues such as these? My
> procedure
> to date has been the following:
>
> a) Optimize a structure in the state of interest (for purposes of
> illustration, let's say lowest triplet state)
>
> b) Take the optimized coordinates and run a TD-DFT calculation on this
> geometry in the ground singlet state. Request that ADF report triplet
> energies.
>
> c) Take the Total Bonding Energy reported from the ground singlet state
> calculation (automatically reported before any TD-DFT is performed) and
> add to this number the vertical excitation reported by TD-DFT for a
> singlet to lowest triplet excitation.
>
> Comments on that procedure?
>
>
> Any comments would be greatly appreciated,
>
> Michael Drummond
> Graduate Student
> Bursten Group
> Department of Chemistry
> The Ohio State University
>

---
Dr. Serguei Patchkovskii
Tel: +1-(613)-990-0945
Fax: +1-(613)-947-2838
E-mail: Serguei.Patchkovskii_at_nrc.ca
Coordinator of Modelling Software
Theory and Computation Group
Steacie Institute for Molecular Sciences
National Research Council Canada
Room 2011, 100 Sussex Drive
Ottawa, Ontario
K1A 0R6 Canada