Subject: excited states

From: [email-masked] (Marketing)
Date: Tue, 21 Jun 94 14:51:46 PDT

Mary Jo Ondrechen wrote:

 |Hello.  We need information on the singlet and triplet
 |excited states of C16H11SN, a highly conjugated system.
 |We are interested in energies and dipole moments of the
 |excited states.  We would appreciate any advice on
 |methods and approach.  I have experience with HF, HFS, and a
 |variety of less sophisticated methods, but all on ground
 |states.  I do not have experience with excited states, and
 |would appreciate advice.

Although ground electronic states are usually well described by
a single electronic configuration, excited states usually are not.
This is another way of saying that many-body effects (electron
correlation) must be included in describing excited state properties.
This is why HOMO-LUMO energy gaps are a poor estimate of actual
UV-Visible spectroscopic energies.
Configuration interaction (CI) is the most common way of transcending the
Hartree-Fock approximation. 

The systems that you describe are probably too large for an ab intio
treatment. However, there are semiempirical treatments that have
been specifically parameterized against UV-Visible transition
energies and perhaps dipole moments. The most widely used method is
based upon the INDO Hamiltonian and is called INDO/S (more recently
appearing as ZINDO/S) and was developed by Michael Zerner's group. 

This method is found in many software packages including
HyperChem and ARGUS. The number of excited states that are to be
included in the CI matrix can be chosen based upon energy or
orbital space criteria. One popular method for excited states
is CIS, where only configuration state functions (symmetrized
combination of determinants) that represent single excitations
>from the reference excited state are included in the CI matrix.
Alternatively, you may increase the active orbital space until you
achieve convergence. Be advised that these calculations will not
treat transitions that carry oscillator strength through vibronic
coupling. On a practical note ... geometry optimizations are
both too hard and probably unnecessary (for rigid systems).

Two experts in this area are John McKelvey (Kodak)
and Mark Thompson (Pacific Northwest labs).

Hope this helps,

Bill

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* William A. Glauser, Ph.D.     |    Internet:  [email-masked]   *
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