Re: [ESPResSo] setmd skin

[Olaf Lenz]

Hi!

On 06/24/2010 08:38 PM, Mikheil Azatov wrote:
> it's obviously not because of it. The reason I change the time step is
> to make the code run faster. For small time-steps the particles move
> just very slow and it doesn't make any sense to keep time-step that small.

Of course it makes sense to keep the time-step that small! The reason is
exactly that the time-step has to be small enough so that the system
doesn't explode.

What happens is the following: when the time-step is too large and two
particles hit each other with high relative velocity, the distance
between the two particles is reduced too quickly, so that they yield in
very high forces and consequently an even higher velocity - which makes
the system explode.

The time-step has to be adapted to the maximal velocity that occurs in
the system and the maximum of the second derivative of any of the
potentials (basically how fast does the force grow). Both are determined
by the system you are simulating, so there is always a maximal time-step
for a given system that you can choose without the system exploding.

Olaf

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