Subject: Transition probabilities using GAMESS

From: "gandalf7" <[email-masked]>
Date: Wed, 19 Feb 1997 13:17:09 -0800

Hello,
As a nascent computational chemist and mere undergraduate, I have
questions that may seem too simple for such an august audience.
However, I shall beg your indulgence and ask anyway.
As part of my senior thesis, I am attempting to calculate the effect
of external electric fields on the radiative transition probabilities
of Nitrogen gas. I know that these probabilities are just the square
of the transition dipole moment.
My questions are:
1. In GAMESS, what does "dipole length" and "dipole velocity",
calculated by "$TRANST", actually mean? Are either one of these the
transition dipole moment used to find the probability?

2. What are the arguments of "$VEC1"? I studied the example provided
with GAMESS on CI transition moments for water, and I cannot figure
out what the numbers represent.

3. Under the "$CIDRT1" keyword, how do I decide on the value of NFZC,
NDOC and NVAL?

I very much appreciate your time and any assistance you can offer.

Thanks in advance,
Lawrence Watts
[email-masked]

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From: "John-M. Sichel" <[email-masked]>
Date: Wed, 19 Feb 1997 21:57:50 -0300 (ADT)

> transition dipole moment used to find the probability?

From memory, they are two different approximations to the transition
dipole moment. Time-dependent perturbation theory leads to an integral of
the operator r(vector), hence "length". This can be transformed (by
integration by parts) into an integral of the momentum operator del, 
referred to as velocity in this context. 

The two integrals are equal for exact wavefunctions of the two states. For
computed approximate wave functions, they are not equal and the difference
between the two may give an idea of the accuracy of the moment.

John Sichel
Universite de Moncton