Subject: Use of MOPAC 6.0 for isotopically-substituted molecules

From: [email-masked] (Hr Dr. S. Shapiro)
Date: Sat, 4 Jun 1994 21:47:04 +0100 (MEST)

Dear colleagues;

     I am a user of semi-empirical quantum mechaical techniques rather than a
quantum mechanist, so perhaps someone will be kind enough to respond to this
request for information.

     (i) I would like to use the AM1 and PM3 modules of MOPAC 6.0 in order to
compare the properties of hydrogen isotope-substituted molecules.  I construct-
ed Z-matrices for three small molecular weight organic molecules which differ-
ed only in that one contained only protium, another contained only deuterium,
and the third contained only tritium.  I ran these in MOPAC under the following
conditions:

                     t=72000 uhf precise am1 nointer ef

The final outputs for all three molecules were identical EXCEPT FOR THEIR MOLE-
CULAR WEIGHTS (which were as they should have been).  Are MOPAC calculations
too insensitive to the impact of deuterio- and tritio-substitution to be of
value, or are there some "tricks" which I must use in order to extract the re-
quired information?

    (ii) In NOT TOO TOO technical terms, what are the definitions of

                  total energy
                  electronic energy
                  core-core repulsion
                  sz
                  s**2

Why is it that for some molecules the ionisation potential (-1 x homo) corres-
ponds to the alpha eigenvalues and for other molecules it corresponds to the
beta eigenvalues?  Why are the alpha and beta eignevalues identical for some
molecules and different for other molecules?

     Thanks in advance to all respondents.

                                            Sincerely,

                                            S. Shapiro
                                            Internet: [email-masked]