Subject: Re: CCL:How are H-bonds parameterized in force fields?

From: John Mitchell <[email-masked]>
Date: Thu, 14 Oct 1999 12:51:01 +0100 (BST)

Well, those of you who have read my thesis probably don't need to read
any further in this reply. In addition to agreeing with Irene, I'd make
the following comments

1) The repulsion doesn't vanish because of some mystical violation of
the Pauli principle. The reasons are:
a) The charge density around polar hydrogens is greatly reduced relative
to non-polar hydrogens, so the charge overlap is reduced, so the
repulsion is reduced
b) Induction and charge transfer effects, which Williams and others can
safely ignore in less polar situations become significant for hydrogen bonds.
So these effects roughly cancel out with some more of the repulsion.

The consequence is that ignoring polar hydrogen repulsion happens to give
a reasonable model in many cases due to cancellation of errors.

2) In general, I don't think there is any need to think of hydrogen bonds
as covalent. They can be described well by perturbation theory, for
instance, and a good representation of non-bonded interactions should
take care of hydrogen bonding automatically. There are some possible
pathological cases, for instance where the symmetry leads to a double well
potential for the hydrogen position. 

                                   John.

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  Dr John B.O. Mitchell,
  Centre for Theoretical and Computational Chemistry,
  Department of Chemistry,
  University College London,         Phone: 020-7679-4665
  20 Gordon Street,                  Fax:   020-7679-7463
  London WC1H OAJ,
  United Kingdom.

  Email address: [email-masked]
  Home page URL: http://www.biochem.ucl.ac.uk/~mitchell/index.html

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