Subject: Re: CCL:Electrostatic model for ordinary hydrogen bonds

From: "Hank D. Cochran" <[email-masked]>
Date: Thu, 21 May 1998 07:32:01 -0400

Dr. Williams:

This subject has received considerable attention since 1991 when we published
results of classical molecular dynamics simulations of water using the
Simple Point Charge (SPC)

  P. T. Cummings, H. D. Cochran, J. M. Simonson, R. E. Mesmer, S. Karaborni,
  "Simulation of Supercritical Water and of Supercritical Aqueous Solutions,"
  J. Chem. Phys., 94, 5606-5621 (1991).

which tested the ability of the model to extrapolate from ambient conditions,
for which it was developed, to supercritical conditions. Hydrogen bonding
in these simulations was explicitly discussed in a subsequent paper

  H. D. Cochran, P. T. Cummings, and S. Karaborni, "Solvation in
  Supercritical Water," Fluid Phase Equilib., 71, 1 (1992).

In 1993, a European group challenged the reliability of the electrostatic
model based on their new neutron scattering experiments at supercritical
conditions using isotope substitution

  P. Postorino, R. H. Tromp, M. A. Ricci, A. K. Soper, and G. W. Neilson, "The
  Interatomic Structure of Water at Supercritical Temperatures," Nature, 366,
  668 (1993).

This paper provoked some controversy, and in 1997, a revised interpretation
of the experimental results was published by the original authors

  A. K. Soper, F. Bruni, and M. A. Ricci, "Site-site pair correlation
  functions of water from 25 to 400 degrees C: Revised analysis of new and
  old diffraction data," J. Chem. Phys., 106, 247 (1997)

based on a different approach to inelasticity corrections of the data. The
revised scattering results are in semi-quantitative agreement with our
original simulations. The erroneous inelasticity corrections affected results
>from prior scattering studies of ambient water and the electrostatic models
based on them. Some early scattering experiments were vindicated by the new
new analyses of old data. This interesting story, with results of simulations
with some new electrostatic models, will appear shortly

  A. A. Chialvo, P. T. Cummings, J. M. Simonson, R. E. Mesmer, and
  H. D. Cochran, "The Interplay Between Molecular Simulation and Neutron
  Scattering in Developing New Insights into the Structure of Water ", Ind. &
  Eng. Chem. Res., accepted for publication (1998).

Regards,

Hank Cochran

On May 19,  2:43pm, Donald E. Williams wrote:
> Subject: CCL:Electrostatic model for ordinary hydrogen bonds
>
> 	There seems to be a widespread misunderstanding that an ordinary
> hydrogen bond (say O-H...O) can be modeled with a purely electrostatic
> potential by "dumping everything into electrostatic parameters".  If anyone
> knows of such a model, I would like to hear about it.
> 	There is a parameterization called the "disappearing hydrogen" model of
> the OHB.  In this model, electrostatics are included, but also (and very
> importantly) the repulsion of the hydrogen atom is set to zero, quite a
drastic
> change from a normal hydrogen atom.  It is easily shown that the effect of
> eliminating hydrogen repulsion is by far more important than the
> electrostatics.
>
> >decompose an HB into its "electrostatic" and "covalent" contributions. An
> >"ordinary" HB (OHB) has both electrostatic and covalent components, although
> >it can be reasonably modelled by classical force fields by dumping
everything
> >into electrostatic parameters. An OHB can be neutral or charged. A charged
>
> -Donald Williams
>
> --
> Dr. Donald E. Williams		email:[email-masked]
> Department of Chemistry
> University of Louisville	phone:502-852-5975
> Louisville, KY 40292		fax:  502-852-8149
>
>
> ---
>-- End of excerpt from Donald E. Williams

-- 
H. D. Cochran 
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Oak Ridge National Laboratory      fax     423/241-4829
Bethel Valley Road                 email   [email-masked]
Oak Ridge, TN 37831-6224 (USA)     http://www.ornl.gov/people/HDCochran
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---

From: Arno Papazyan <[email-masked]>
Date: Thu, 21 May 1998 06:51:29 -0700 (PDT)

Heh, the "the disappearing hydrogen" is a fine technical point. However,
the electrostatic effect of the HB would be a nice, round zero without the
electrostatic parameters. **That** is the important effect of the HB. The
disappearing hydrogen can be more fairly described as part of the
force-field model, and is indeed a good way of taking the covalent part of
the HB into account. Just like SHAKE is a good way of acknowledging that
bonds exist between atoms without actually doing its QM.
Regards,
Arno Papazyan  

On Tue, 19 May 1998, Donald E. Williams wrote:

> importantly) the repulsion of the hydrogen atom is set to zero, quite a drastic
> >it can be reasonably modelled by classical force fields by dumping everything
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