AMBER Archive (2008)

Subject: Re: AMBER: calculating the enthalpy of evaporation

From: David A. Case (
Date: Thu Jul 24 2008 - 15:31:01 CDT

On Thu, Jul 24, 2008, jessica stolee wrote:
> I would like to calculate the enthalpy of evaporation for a water molecule
> from a box of water molecules and am not quite sure where to begin. Would I
> take one molecule and let it extend to infinity and subtract the energy of
> that from the energy of the box of water molecules? If anyone could give me
> some useful hints I would appreciate it.

Take a look at these two papers:

%A H.W. Horn
%A W.C. Swope
%A J.W. Pitera
%A J.D. Madura
%A T.J. Dick
%A G.L. Hura
%A T. Head-Gordon
%T Development of an improved four-site water model for biomolecular
simulations: TIP4P-Ew
%J J. Chem. Phys.
%V 120
%P 9665-9678
%D 2004

%A H.W. Horn
%A W.C. Swope
%A J.W. Pitera
%T Characterization of the TIP4P-Ew water model: Vapor pressure and boiling
%J J. Chem. Phys.
%V 123
%P 194504
%D 2005

The first gives a very clear explanation of how to calculate the enthalpy of
vaporization. Basically, you start with <U>/N, where <U> is the average
potential energy and N is the number of water molecules in your box, and then
apply some corrections to make comparison to the experimental number.

The second paper deals with calculating the free energy of vaporization; this
is not what you asked about, but it is interesting to compare and contrast the
two problems.

...good luck...dac
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