Subject: High level for TMs

From: Per-Ola Norrby <[email-masked]>
Date: Mon, 4 Feb 2002 11:39:27 +0100

Dear CCLers,

	I'd like to get some opinions on high level calculations on 
first row transition metal complexes, in particular for complexes 
with near-degenerate states.  Furthermore, I'm interested in 
complexes used for homogeneous catalysis, so even very smal models 
frequently have 10-20 heavy atoms and as many hydrogens in addition 
to the metal.

	I've used various types of DFT, both pure and hybrid, for 
these types of systems for several years now.  However, I've seen 
several indications recently that the relative energies of different 
spin surfaces can be wrong by up to 100 kJ/mol.  This is, of course, 
unacceptable.  I very much like the pragmatic fix proposed by Reiher 
et al, Theor. Chem. Acc. 107 (2001) 48-55, where they reparameterize 
the amount of exact exchange in B3LYP for organometallic complexes. 
However, having been burned once, I don't want to assume that one 
fixed parameterization is going to be valid for all first row 
transition metals.  Thus, to apply this approach I need very good 
reference data for some experimentally inaccessible complexes.  I'll 
have to obtain them computationally, but what level to use? 
Alternatively, I may not go for the modified DFT, but then I still 
need a theoretical level I can access, which will give me good 
relative energies (and the geometries, but maybe DFT can do this..?).

	So, what level can we trust for this?  CCSD(T) has been 
proposed, but it's expensive, and we all know that it requires a 
large basis set, certainly much better than the double-zeta valence 
that is often used.  And it is still only a perturbation method, so 
if the underlying HF is way off, no amount of perrturbation will fix 
it.  However, I'd like to try it out if I can find a program that is 
reasonably efficient.  I've heard rumors of linear scaling CCSD(T) or 
equivalent methods, does anyone know of an available implementation 
that will allow a calculation with ca 500 basis functions?

	How about MC methods?  I'm trying out CASPT2, but what else is there?

	BTW, don't suggest any MP-method (MP2, LMP2,...).  I've tried 
them for a couple of test cases, and the errors are much larger than 
with DFT.

	As a side issue, any suggestions of what type of experimental 
data to compare to?  There are a couple of cases where population 
distributions among near-degenerate states can be measured, but this 
is not general enough, I also want to be able to predict cases where 
the energy difference is large.

	All the best

	Per-Ola
-- 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Per-Ola Norrby, Assoc. Professor, http://compchem.dfh.dk/PeO/
Technical University of Denmark, Dept. of Chem., Org. Chem.
Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark
Email: [email-masked]  tel +45-45252123,  fax +45-45933968