[gmx-developers] Use of invsqrt with tables

[Mark Abraham]

Hi,

This isn't related to using tables per se, but rather with using potentials
that lack singularities that prevent r=0 in practice. I think such kernels
should rather use an approach like that we use for exclusions:

rsqmod = (rsq == 0) ? smallnumber : rsq;
rinv = invsqrt(rsqmod);
rinv = (rsq == 0) ? 0 : rinv;

and maybe a mask on energies also.

gmx::sqrt in the SIMD module is also an option that implements the same
kind of conditionality, but doesn't solve the need to have a functional
rinv.

Mark

On Wed, Mar 23, 2016 at 1:42 PM David van der Spoel <spoel at xray.bmc.uu.se>
wrote:

> On 23/03/16 13:31, Berk Hess wrote:
> > Hi,
> >
> > But avoiding the inversion is not possible, since you need to multiply
> > the scaling force by the normalized distance vector.
> The force should be zero at r=0 for symmetry reasons and hence the code
> should not crash.
> >
> > Berk
> >
> > On 2016-03-23 13:00, David van der Spoel wrote:
> >> Hi,
> >>
> >> In all versions of gromacs the distance between two nonbonded atoms is
> >> computed as
> >>             rinv00           = gmx_invsqrt(rsq00);
> >>             r00              = rsq00*rinv00;
> >>
> >> This is fine when we use Lennard Jones and Coulomb, but not when using
> >> table potentials which do not necessarily have singularities at r = 0.
> >> I would therefore propose to replace the statements by a normal sqrt
> >> in all innerloops using tables.
> >>
> >> Thoughts?
> >
>
>
> --
> David van der Spoel, Ph.D., Professor of Biology
> Dept. of Cell & Molec. Biol., Uppsala University.
> Box 596, 75124 Uppsala, Sweden. Phone:  +46184714205.
> spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
> --
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