Subject: MOPAC 93: bug report may be premature

From: [email-masked] (Dr. James Stewart)
Date: Fri, 3 Nov 95 08:06:49 PST

Bug in MOPAC 93 ?

Yesterday I reported what might be a bug in the calculation of the
transition dipole in MOPAC 93.  This report resulted in several people
telling me that the ground-state dipole is <psi_g|er|psi_g>, and that
obviously can be non-zero.

The potential bug was reported because users have a right to believe
that the software they are running does in fact work correctly.  Perhaps
I `jumped the gun' in announcing a bug without doing more homework.  

Now for an update.  All tests on the transition dipole have failed to
show that a bug does in fact exist.  However, and here is where I need 
help, there is a point of theory that I don't understand.  That is, for 
a transition dipole, which of the two following statements is true:

    1:   <psi_i|er|psi_j> = +<psi_j|er|psi_i> 

    2:   <psi_i|er|psi_j> = -<psi_j|er|psi_i> 

where psi is an atomic orbital, a molecular orbital, a microstate,
or a state function.  Yes, I know this looks trivial, but for just now 
it's got me confused.  MOPAC 93 uses statement 1.

In the meantime, the available evidence is that the transition moment
calculation is correct, within the approximations used.  If statement 2
is correct, then the transition dipoles will be in error by a few percent.

Any comments on this would be appreciated.  

James J. P. Stewart