Subject: ECPs, TMs and energetic data

From: [email-masked]
Date: Wed, 4 Nov 92 17:21 CDT

I think Jan Hrusak raises an interesting (and somewhat disturbing)
question - what is the reliability of ECP/valence basis set schemes for
energetic predictions.  I think we can agree that as long as the correct
wavefunction and a suitably flexible basis set is used that geometric
prediction is quite good.  But what about energetic data?
        Recently, Walt Stevens and I have finished ECP derivation for the
lanthanides and comparing numerical, Dirac-Hartree-Fock calculations with
ECP calculations suggests that the biggest discrepancies come about due to
the valence basis set and not the ECP approximation.  This is somewhat
encouraging since it is easier to augment the basis set than rederive the
ECPs.  Preuss, Stoll and co. have published a paper, also dealing with the
lanthanides, where they look at the ordering of atomic states as a function of
ECP core size.  They conclude that one should probably use a very small core
size (I think it was either Ar or Kr) if one is interested in reproducing
this sort of data (at least this is my interpretation of their conclusion).
For the TMs, it was essential to include ns and np to reproduce energy
ordering of atomic states; I believe that all workers in this area have
made this conclusion.
        Although ordering in these coordinatively unsaturated systems is
very important given the bonanza of thermodynamic data to emerge from
beam, FT-MS and FT-ICR techniques, another question is what about coord.
saturated complexes?  For main group and organic systems, the RHF geometry/
MP2 energy scheme seems to be quite effective, is anyone making a systematic
study for TM systems?  Frenking, Gauss et al. have shown the energy ordering
of isomers in d0, six-coord. complexes to vary widely with correlation level.
However, we get good quantitative agreement with activation barriers for d0
complexes (admittedly a best case scenario) using a simple RHF/MP2 scheme.
I should point out that in all cases the experimental folks gave us the
experimental numbers after we told them the calculated values, which is
somewhat satisfying!  I would be very interested to hear from netters what
their experience has been on predicting enthalpic data for TM systems using
various correlation schemes.

                                                Tom Cundari
                                                Assistant Professor
                                                Department of Chemistry
                                                Memphis State University
                                                Memphis, TN 38152
                                                phone:901-678-2629
                                                fax:901-MSU-EIGS