Subject: SUMMARY : how to calculate I.P. of large compounds

From: Didier MATHIEU <[email-masked]>
Date: Tue, 20 Mar 2001 18:28:34 +0100

Thanks for the advice and pointers concerning methods for efficient
evaluation of chemical potentials.

I am especially grateful to Stefan Grimme for pointing out that the
Delta SCF method using B3LYP should yield I.P.s within 0.5 eV or better
with extended basis sets. I had tried this approach before but obtained
poor results owing to my lack of experience with the Jaguar program.
Indeed, even taking care of the symmetry of the wf, the SCF iterations
for the cation doublet often lead to spurious excited states. I have
tested only few of the many options for the initial guess, however, to
converge systematically to the cation ground state, I had to use the
following procedure:
1. calculations on the molecule using only Abelian point group symmetry.

2. calculation on the doublet cation using the resulting orbitals from
the molecule as a initial guess, and again allowing only Abelian point
group symmetry assignment.
The restriction to Abelian symmetry in step 1 is useful in view of
reusing the orbitals as initial guess in step 2, otherwise an excited
state was found in few cases.
Out of test calculations on a set of 12 aromatic molecules, I got
average and maximum errors of resp. 0.12 and 0.30 eV at the
B3LYP/6-311G**//B3LYP/6-31G** level.

With regard to Green fct techniques, I was first led to consider
OVGF methods in combination with semi-empirical hamiltonian that have
been implemented, apparently in a commercial version of MOPAC.
However, the use of MNDO-type hamiltonians appear to have a significant
impact on the accuracy: I recently received P. Diet et al. J. Chem. Soc.
Perkin Trans 2 (8) 1653 (1999) where such calculations are reported. The
errors of this approach to I.P.s, although 40% < that obtained through
Koopmans theorem, are c.a. 3x > that obtained through Delta SCF / B3LYP.

Regards,

--
Didier Mathieu
CEA - Le Ripault
BP 16
37260 Monts
+33 02 47 34 41 85