Subject: Units and parameterization

From: Howard Alper <[email-masked]>
Date: Tue, 01 Sep 1998 16:22:08 -0500

Hello,

  I have a question concerning the parameterization of simulation
forcefields, the size of the parameters, and the units.

  Forcefields are parameterized such that one gets energies in
kcal/mole (or kj/mole) even though one is simulating a number of
particles much less than Avogadro's number, so that the actual
energy is more like whatever number of kcal/mole/6.02 * 10 ** 23,
for example.

  As some examples of consequences of this way of parameterizing
forcefields, consider a simulation of N water molecules (my favorite!)
If one want to compare to the experimental potential energy (in
kcal/mole), one does not use the total energy of the N molecules
in the simulation, but the total energy/N.  Likewise, if one wanted
the heat capacity, that would involve differences of fluctuations
in the energy/particle, not the total computed energy of the system of
N molecules.

  My question is: does anyone know the exact reasons for or derivation
for the above methods (conventions?) of calculating properties from
simulations?  I am trying to explain this to someone who is very new
to simulation, but it seems like I "know what to do" by some process
of osmosis, and I do not remember being taught directly how to
calculate things like E or Cp, so I end up saying " I just know".
I am sure that there is a rational reason or process behind what
is done, I just can't make it clear.  Any explanations or references
would be most welcome.

  Thanks,

  Howard

-- 

Howard E. Alper, Ph.D.
Dept. of Chemistry and The Advanced Materials Research Institute
University of New Orleans
New Orleans, LA 70148
504-280-7216

 - Helping molecules find happiness for almost a 5th of a century.