Re: Question on Ewald and Belly!

From: Thomas A. Darden <>
Date: Mon 12 Jul 1999 10:25:54 -0400 (EDT)

Dear Dat,
Actually its not serious. The error has to do with the forces not being
correctly related to energies. With the belly option and a cutoff, you
specify which nonbond pair forces and interaction energies are
calculasted, and so the energies and forces are consistent. However,
with PME the reciprocal energy cannot be calculated pair by pair. So, with
PME and belly, the direct sum is correctly filtered, but the full
reciprocal energy for the whole cell is calculated. Afterwards the forces
on the frozen atoms are zeroed. So, the forces on atoms are correct, but
the reported energies are not correct. Since the reciprocal energy is
typically small compared to other energies, the error is small. However,
the important thing is the dynamics, which is correct.
Hope this helps,
Tom darden
ps. I don't recommend constant pressure with belly option. If you want
constant pressure, I would instead use cartesian constraints. Or at least
use molecule based scaling with constant pressure. However, others may
have other opinions about this, depending on their experiences.

On Sun, 11 Jul 1999, Dat H. Nguyen wrote:

> Dear AMBER Users:
> I got the following warning message at the beginning of the simulation:
> Warning: Although EWALD will work with belly
> (for equilibration), it is not strictly correct!
> (iewald = 1)
> Can anyone tell me how serious it is in terms of the stability of the
> trajectory, energy and the quality of the numerical results. I'd like to
> use both PME and Belly option for the production stage of my
> simulations.
> Thanks!
> Dat
Received on Mon Jul 12 1999 - 07:25:54 PDT
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