Re: CCL:Electrostatic model for ordinary hydrogen bonds

 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:dew01(-(at)-)xray5.chem.louisville.edu
 > 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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