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Re: WSN: Water RDFs and thermodynamic properties (RE-SENT) (fwd)
Sender: bruce_bush@merck.com (Bruce Bush)
Subject: Re: WSN: Water RDFs and thermodynamic properties
Yes, thermodynamic properties can be determined approximately from pairwise
radial distribution functions. Lennard-Jones is just a special case, not
the basis of the method. The usual approximation is called "pair
decomposable": that the Hamiltonian (the energy-function, not the
thermodynamic quantities) can be decomposed into one and two-body terms such
as Lennard-Jones + Coulomb, or any other atom-pair function.
Then the internal energy and all the free energies and other thermodynamic
quantities -- pressure, entropy, etc. -- depend only on the two-body
distribution function for each pair of species.
[The averaging over the MD / MC simulation that goes into this distribution
function takes account of 3-, 4-, and more-body effects.] A brief intro is
David Chandler, _Intro to Modern Stat Mech_ (Oxford, 1987) pp 201 ff.
The papers recommended by others in WSN give details for water.
All kinds of papers on "potential of mean force" and "excess thermodynamic
quantities" address the issues of one or two solute particles in the fluid.
Notice that the RDF of a pair of atoms or a pair of species depends
on the potential energy functions for all species, not just for that
pair. The H-H distribution, for example, depends greatly on the interactions
between O and O. So the thermodynamic quantities can't be decomposed
into a function of JUST the H-H potential energy, plus a function of the
H-O potential, plus ... Too bad!
Best wishes --
/ Bruce /
bruce_bush@merck.com
>
> Greetings! This is my first posting on this board; I hope I sent it to the
> right address. My question is, can the atomic, i.e., O-O, O-H, and H-H
> distribution functions for water which are available from MD simulation
> be used to calculate any thermodynamic properties, like can be done for
> simple Lennard-Jones sytems? I have never been able to locate any
> equations or record of anybody doing this. I would appreciate any insight
> anyone can give me.
>
> Thanks, rpratt@mines.colorado.edu
>
>
>
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