Subject: Re: CCL:Energy conservation of molecular dynamics in the presence of electrostatic forces

From: Konrad Hinsen <[email-masked]>
Date: Thu, 6 Mar 2003 18:11:12 +0100

On Thursday 06 March 2003 15:20, you wrote:

> Problems, however, arise when molecules are charged. Because
> electrostatic forces are much more long-ranged (1/r dependence) and
> typically stronger, the magnitude of electrostatic forces on atoms can
> be several hundred (or even thousand) times greater than that of van
> der Waals forces. As a result, the time step has to be accordingly
> reduced for two or three orders of magnitude, in order for the total
> energy to conserve. This means a time step of 0.001-0.01 femtosecond
> has to be applied.

That doesn't seem quite right. A long range does not imply a strong force, it 
just means that the force on any given atom depends on the positions of many 
others. The contributions can (and often do, in practice) cancel out. 
Moreover, large forces do not automatically imply fast motions, other 
parameters (e.g. temperature) enter as well.

You don't say what kind of systems you study. In liquids and biomolecules at 
room termperature, a time step of 1 fs works perfectly well with 
electrostatic interactions. One does see improvement of energy conservation 
when going to 0.5 fs, but there is no need to go to time steps as small as 
those you quote. So either your system is very different, or there is a bug 
in your force calculation code.

> Energy conservation is generally not an concern for most classic molecular
> dynamics simulations that assume a heat bath (for controlling the
> temperature to a desired value, thus remove the numerical errors resulted
> from large a*dt^2).

That depends on the thermostat that is used. Personally I prefer the extended 
systems method, precisely because there is still a conserved energy that can 
be used to monitor the behaviour of the simulation.
-- 
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