Re: AMBER: cut-offs in non-periodic simulations

From: Andreas Svrcek-Seiler <>
Date: Mon, 3 Oct 2005 21:15:19 +0200 (CEST)

> Does it mean that it is not possible to truncate Coulombic
> interactions with for exemple a cubic spline
> when running non-periodic simulations ? What would be the reason for not
> doing so ?

As for the reason for not doing so: Just plot E_coul(r) for a pair of
charges (e.g. monovalent ions if you like units) and smoothly truncate
that (graphically, no calculus is needed). Then, again graphically, sketch the
(absolute of the) corresponding force - the negative derivative of
E_coul. Now you see a strong force in the splined (=smoothed,
interpolated) region. This is the reason - it is unphysical and worse than
the ugly 'hard' cutoff.

good luck

            ( O O )
               o Wolfgang Andreas Svrcek-Seiler
               o (godzilla)
       .oooO Tel.:01-4277-52733
       ( ) Oooo.
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Received on Mon Oct 03 2005 - 20:53:00 PDT
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