[gmx-users] Tabscale

[Yuguang Mu]

Dear David,
Generally speaking, you are right.
But in my case, not !
I use -rerun switch, it is exactly the same trajectory.

I check the different energy terms, only the Coulomb (LR) (using PME)
term differs evidently by 0.5% , but systematicly , the single precision
values always larger (or smaller )  than double one by around 0.3-0.5% .
Here I show two figtures about this terms coming from two differnt
systems.

You can see clearly that
they comes from the same trajectory, but differ a certain amount.

So it seems that the PME term differs a factor of 0.3-0.5% .

can we improve it ?
At least this Coulomb (LR) should be as good as other terms
having a difference factor of 0.0003-0.0005% which the single precision
promises.




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 17 Sep 2003, David wrote:

> On Wed, 2003-09-17 at 12:37, Yuguang Mu wrote:
> > 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 ?
>
> I don't think so. It is just lack of precision which leads to another
> trajectory. If you have anisotropic scaling you must increase tau_p to
> at least 5 ps. Even then two different trajectories may give you
> different final box size. Try e.g. to run two double precision runs with
> different starting velocities and see how the box evolves.
>
> >
> > 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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> --
> Groeten, David.
> ________________________________________________________________________
> Dr. David van der Spoel, 	Dept. of Cell and Molecular Biology
> Husargatan 3, Box 596,  	75124 Uppsala, Sweden
> phone:	46 18 471 4205		fax: 46 18 511 755
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