Subject: internal vs. external

From: "Dr. Richard L. Wood" <[email-masked]>
Date: Wed, 01 May 2002 15:01:13 -0400

Hi all

In CHARMM, when the virials are calculated (and from these, pressures),
what is meant by the terms "EXTERNAL" and "INTERNAL"?

We've noticed that if you run a dynamics calculation, and you input a
volume,
the output gives you an "EXTERNAL" and an "INTERNAL" pressure (PRESSE
and PRESSI, respectively) if you use periodic boundary conditions, and
if you
don't use PBC, the EXTERNAL pressure term is 0 in some cases.

We are trying to understand and rationalize these pressures that we are
seeing
(only if we input a volume), and they just don't make any sense to us,
nor do the
terms EXTERNAL and INTERNAL.

We will appreciate any help.

Richard

--
Richard L. Wood, Ph. D.
Physical/Computational Chemist
Post-doctoral Associate
Department of Food Science
120 Stocking Hall
Cornell University, Ithaca, NY 14853