[gmx-users] Energy Conservation with 4fs timestep

[Berk Hess]

> Date: Wed, 1 Apr 2009 14:15:05 +1100
> From: Mark.Abraham at anu.edu.au
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] Energy Conservation with 4fs timestep
> 
> Joe Joe wrote:
> > Hi,
> > 
> > I get good conservation when running NVE in gromacs with 4 fs when I use 
> > PME-switch for electrostatics but not so good when I use switch. Any 
> > thoughts why that would be? Params shown below.
> 
> Finite cutoffs (such as used with "switch") are intrinsically unlikely 
> to conserve energy.
> 
> Mark

That is not right.
Finite cut-off's such as switch and shift are purposely designed to conserve energy.
Switch is not particularly good though, since the switching introduces large forces.
I would advise to use PME-switch or reaction-field-zero for electrostatics and shift for vdw.

But the main problem in your setup seems to be the 0.1 nm buffer between the cut-off
and rlist. In general you will need a buffer of 0.25 to 0.3 nm.
You can use rlist=-1 to get exact integration and then vary rlist to get a reasonable
neighborlist update frequency (somewhere around 10 steps).
We should automate the choice of rlist such that the user does not need to worry
about this.

Berk




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