Subject: ECP calculations

From: Prof. Dr. Helmut Schwarz <[email-masked]>
Date: Wed, 4 Nov 92 15:10:39 +0100

To initiate a discussion about the reability of ECP by TM calculations
i have summarized some results for the iron meutral an ion

          5D->5F     5D->3F    5D->6D     5D->4F
----------------------------------------------------------
SCF (UHF)
ECP(Ar)   1.45       2.08       6.68      7.71
ECP(Ne)   1.86       3.37       6.71      8.12
PP(Ar)    1.87       4.49       6.32      7.30
rel AE    2.06       7.53       6.29      8.27
----------------------------------------------------------
MP2
ECP(Ar)   0.50       1.11       6.89      6.66
ECP(Ne)   0.67       1.25       6.90      7.19
PP(Ar)    0.87       1.42       7.46      8.09
----------------------------------------------------------
exp       0.86       1.49       7.86      8.09
----------------------------------------------------------

in the table are given the calculated excitation energies for the 
iron (in eV) by using the ECP of Hay with a argon and neon core
respectivelly together with a pseudopotential (Durand&Barthelat)
parametrized by us. The basis sets used are ECP(Ar)-DZP, for the 
ECP(Ne) the contraction of Frenking (TZP) and for the PP calculations
we use 6s3p8d/5s3p5d.

I think, that the results clearly demonstrate, that the ECP(Ar)DZP 
are not able to describe the excitation energies even in a qualitative
way (6D X 4F ). So, as decribed by G. Frenking a expansion of the 
valence space and a more flexible basis can be used to make the results
more realistic. But as can concluded from the table, the results remain
still not very satisfying. In my view this problems are connected with
ground state oriented parametrization procedure more than with the
valence space. Our PP paramatrization (in fact very similar to Hay) use
the larger core and the agreement with all electron (AE) and experimental
values is rather good.

The problem of proper description of the excitation energies is very
important also in low spin (coordinative saturated TM systems) because
the electrinic valence state in a M-(L)n system is often diferent from
the atomic ground state. Errors in the description of the excitations
at the atomic level will lead to uncertainty of at least +/- 10 kcal/mol
in molecutr systems.

The message is be carefull if you are using the ECP of Hay (especially
in the GAUSSIAN program series, where they often converge to a excited
state.

Jan Hrusak