B3LYP models electronic and repulsive steric effects quite well (although I would go for some other functional when having transition metals). But as most of the DFT methods, it does not work well with dispersive forces. If you add a dispersion correction add on (for instance check: http://jcp.aip.org/resource/1/jcpsa6/v132/i15/p154104_s1 ) the results will be much better (look at http://pubs.rsc.org/en/Content/ArticleLanding/2010/DT/C0DT00778A ). But, if you work with a QM/MM type of method, do not add dispersion forces. You should choose an outer layer that can manage dispersion forces by itself, (like an MM method).
MP2 may be OK, but not advisable with transition metals. You can forget CCSD, unless you have a quantum computer.
Now, one of the biggest problem with the separation of steric and electronic effects is that you will loose accuracy everywhere, and you will have to work with qualitative values. One of the problems will be the solvation model, that does not work with ONIOM (unless there is some update I am not aware off). And you will have lots of artifacts from the separation scheme.
You may want to have a look at J. Mol. Cat. A 2010 , 324 , 120-126. In there we tried to tackle some of those problems.

Best,

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..........Sebastian Kozuch...........
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....Weizmann Institute of Science....
...........Rehovot, Israel...........
.. sebastian.kozuch#,#weizmann.ac.il ..
http://yfaat.ch.huji.ac.il/kozuch.htm
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From: Michael Dimitrov mich.dimitrov^^gmail.com <owner-chemistry#,#ccl.net>
To: "Kozuch, Sebastian " <kozuchs#,#yahoo.com>
Sent: Wed, November 10, 2010 9:08:22 PM
Subject: CCL: Separation of steric and electronic effects

Sent to CCL by: Michael Dimitrov [mich.dimitrov,+,gmail.com]
Hi,

I am interested in the procedure of computational separation of steric
and electronic effects as described in Scheme 3 of the article Eur. J.
Inorg. Chem. 2007, 5390 ( http://dx.doi.org/10.1002/ejic.200700850 )

I doubt if B3LYP level is good enough to take into account steric and
electronic factors? Perhaps it would be better to use MP2, CCSD or
another density functional for accurate description of both steric and
electronic factors?

Best regards,
Michael Dimitrov.

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