[gmx-users] Tabscale

[Yuguang Mu]

Dear Erik,
The question I really want to ask is that why the virial calculated from
the same trajectory but using different precisions are quite different.

I have a trajectory simulated in single precision, then I use -rerun
switch to recalculate energies in double precision.

The potentials, kinetics, terms turn out nearly exactly same, but I found
the virial, also, the pressure, specially the off-diagonal terms, deviate
greatly.

In other words, if we eqilibrate the system in single precision with
pressure coupling (orthogonal box but with anisotropy coupling), the
final shape of the box will be greatly different from that simulated with
double precision using the same protocol.

I donot think it tolerant.

Is it possible that in the formula of virial calculations there happens
somehow overflow inthe case of single precision ?

Dr. Yuguang Mu
Institute for Physical and Theoretical Chemistry
J.W. Goethe University Frankfurt am Main
Marie Curie Str. 11
60439 Frankfurt/Main, Germany
Tel: +49-(0)69-798-29711

On Tue, 16 Sep 2003, Erik Lindahl wrote:

> Hi,
>
> Not without hacking the code. The tabscale is selected as 500 points/nm
> in single and 2000 points/nm in double. With the cubic spline
> interpolation, this essentially gives you full single precision
> accuracy, and almost full double precision accuracy for all common
> potential forms, respectively.
>
> Cheers,
>
> Erik
>
>
> On Tuesday, September 16, 2003, at 09:26 AM, Yuguang Mu wrote:
>
> > Hi All,
> > Inthe log file always printed out the following lines:
> >
> > Table routines are used for coulomb: TRUE
> > Table routines are used for vdw:     FALSE
> > Using a Gaussian width (1/beta) of 0.320163 nm for Ewald
> > Cut-off's:   NS: 1   Coulomb: 1   LJ: 1
> > Generated table with 2000 data points for COUL.
> > Tabscale = 2000 points/nm
> > Generated table with 2000 data points for LJ6.
> > Tabscale = 2000 points/nm
> > Generated table with 2000 data points for LJ12.
> > Tabscale = 2000 points/nm
> > Generated table with 3200 data points for Ewald.
> > Tabscale = 2000 points/nm
> > Generated table with 3200 data points for LJ6.
> > Tabscale = 2000 points/nm
> > Generated table with 3200 data points for LJ12.
> > Tabscale = 2000 points/nm
> >
> > here i sthe question, could I change the Tabscale manually ?
> >
> >
> > Dr. Yuguang Mu
> > Institute for Physical and Theoretical Chemistry
> > J.W. Goethe University Frankfurt am Main
> > Marie Curie Str. 11
> > 60439 Frankfurt/Main, Germany
> > Tel: +49-(0)69-798-29711
> >
> >
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