Subject: Re: CCL:TM force fields

From: Per-Ola Norrby <[email-masked]>
Date: Thu, 10 Aug 2000 19:39:26 +0200

Hi all,

	I'd like to take up this thread again.  Also, I have a 
question at the end:

Isaac B. Bersuker wrote:

>The open (or quasi-open) d shell is the main reason of peculiar 
>properties of  TM systems (TMS)
>with regard to modeling:
>
>(1) 3D delocalization and hence strong mutual influence of  the M-L 
>bonds on each other (e.g.,
>cis- and trans-effect),
>(2) "back donation" and hence local excitation of the ligands 
>(chemical activation by
>coordination),
>(3) degeneracy and pseudodegeneracy and hence the strong Jahn-Teller 
>and pseudo JT effects.
>
>All this together makes imposible to get reasonable parametrization 
>of a force field (with
>transferable parameters) that could be used in modeling similar to 
>that of organic and/or
>non-TMS, in general.
>
>I am sorry that these important features of TMS remain unknown to 
>people engaged in their
>investigation.

	I completely agree that these effects can make modelling of 
transition metals challenging.  However, I disagree strongly with the 
word "impossible".  We are still far from a force field that can do 
this for all conceivable complexes without parameterization, but wide 
transferability is not a requirement for force field modelling. 
Personally, I design force fields for specific problems (you can 
check my web page for examples, see below).  Setting up the force 
field requires about the same time as doing e few QM/MM calculations, 
then I have access to a tool that allows rapid conformational 
searching.  In the types of problems I'm working with (asymmetric 
catalysis), a good sampling of the conformational space is essential. 
The force fields are still 3-4 orders of magnitude faster than QM/MM 
calculations in this case, and seemingly almost as accurate.  An 
attractive combination is to use the force field as a search tool, 
followed by refinement of a few selected structures by QM/MM.

	For the concerns listed above, we've only done simple things 
like introduction of specific cross terms to reproduce trans 
induction, but others have done a lot more.  I'm aware of work by 
Landis, Comba, Deeth, and Carlsson, solving different aspects of 
these problems.  I would like to see more.  If you could send me 
references on how to treat the above problems, I'll summarize to the 
list.

	Best regards,

	Per-Ola
-- 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  *  Per-Ola Norrby, Associate Professor
  *  The Royal Danish School of Pharmacy, Dept. of Med. Chem.
  *  Universitetsparken 2, DK 2100 Copenhagen, Denmark
  *  Tel.   +45-35306506, fax +45-35306040
  *  Email: [email-masked] (preferred), [email-masked]
  *  WWW:   http://compchem.dfh.dk/PeO/