Subject: your Mn and Cr complexes,

From: Philippe Maitre <[email-masked]>
Date: Wed, 30 Apr 1997 18:55:26 +0200

Dear Michael,

        I have had similar problems and I guess I could help you.
You said "I assume the fact that Mn(CO)3 is positively charged has nothing
to do
with it (it is a closed shell)". I do not agree.

        The typical problem of SCF convergence with Transition Metal is
that you have a near generacy of several d orbitals for low-coordination
complexes. This leads to a near degeneracy of electronic configurations,
which therby leads to a near degeneracy of determinants. And that's your
problem in your iteration.

        This problem can be further complicated, sometimes, by pseudo
potential. I do not have any experience with Gamess-UK, but with a
Gaussian92 or Gaussian94, the internal guess of orbitals (either generated
by diagonalizing HCORE or by using the extended Huckel approach) is
extremely
bad when using Pseudo-potential. Do not ask me why, I do not know. Just
one more thing : this bad initial guess obtained when using a pseudo
potential
does not mean at all that the pseudo potentials are bad.

        There is a simple trick to converge an scf calculation with
low-coordinationtransition metal system. You take a piece of paper and you
find out the orbital
energy level. For ML3 systems, you can look at "Orbital interaction in
chemistry"
by Albright, Burdett and Whangbo. Then, you find out a spin state (with
the
same numebr of electrons or not) which leads to a large energy difference
between
the Ground state electronic configuration and the first electronic one
(i.e.
where there is a large gap between the HOMO and the LUMO).
        Run an ROHF or RHF if this is a closed shell on this guy. This
will converge
easily. One it's done, reuse the converged orbitals of this calculation to
calculate your system of interest.

        Good Luck,
        If you have any problem, do not hesitate to contact me. But
please,
give me a geometry and a spin state.

        Philippe Maitre
        Laboratoire de chimie theorique
        Batiment 490
        Universite de Paris XI
        91405 Orsay , FRANCE
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