Subject: What is really a hydrogen bond?

From: "Merethe Sjovoll" <[email-masked]>
Date: Fri, 25 Jul 1997 13:34:58 -0600

Hello everobody,

I am interesting in sharing any experience people have with mimicing hydrogen
bonds in forcefields. The main contribution to an O...H bond is obviously the
coulombic interaction. The other thing is dispersion.
The coulombic interaction is usually represented by two point charges and
dispersion by a -C/R**6 term in a forcefield - I have also seen the term
R/R**10
used.
 However, the hydrogen bonds also have a directional character which is not
described by this simple model.
Are really forcefields of the form described above generally able to describe
structures with hydrogen bonds?

I would very much like to know the view of my colleagues on what a hydrogen
bond really is made of and what it takes to describe it.

Best wishes

Merethe

-- 

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Merethe Sjovoll, Ph.D.                      *          *
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Norsk Hydro a.s Research Center             *          * 
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From: Per-Ola Norrby <[email-masked]>
Date: Fri, 25 Jul 1997 15:34:27 +0200

>structures with hydrogen bonds?

	Several force fields get good results with this simple model.  You
do get some directionality from the electrostatics, irrespective of whether
you use an X-H dipole or partial negative charge on X and positive charge
on H (I find it easier to visualize the directionality with the dipole
model, but the result is the same with point charges).  Other force fields,
like (original) MM3, add specific directionality terms (e.g. a cosine
function).  I'm not sure whether this actually improve the results, I
believe it's much more important to have a well-balanced electrostatic
picture.
	Yet another way to get a better picture might be to include
geometry-dependent polarization of the charge, but not many available force
fields have this feature.  I believe you get something like it in the
Cerius2 implementation of UFF (a "charge equilibration"), but I'm not
impressed with the results.

	Best regards,

	Per-Ola Norrby

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Per-Ola Norrby, [email-masked]
Royal Danish School of Pharmacy, Dept. of Med. Chem.
Universitetsparken 2, DK-2100 Copenhagen, Denmark
Tel: +45-35376777-506, +45-35370850, fax +45-35370922

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From: A J Turner <[email-masked]>
Date: Fri, 25 Jul 1997 15:40:19 +0100 (BST)

Hi!

I have used the 'point charge' force fields to mimic hydrogen bonds.  To
get the pure molecular mechanics force fields to mimic the more accurate
qm/mm force fields (e.g. MP2/6311g** oxgyen and TIP3P hydrogen) requires
the use of n+1 point charges to describe the oxygen, where n= number of
loan pairs.  Single point models lack the directional nature in general -
but can have it is specific cases.  e.g. TIP3P is directional due to the
interaction of the charges on the protons and that on the oygens.
Carbonyls etc are not directional.

Best wishes

Alex 

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|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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