[gmx-users] Long Range dispersion correction with Lennard-Jones PME

[David van der Spoel]

On 20/07/16 17:01, Joel Jose Montalvo Acosta wrote:
> Dear Gromacs users and developers,
>
> I want to understand how the long-range dispersion correction is
> implemented in gromacs when van der waals (vdw) interactions are computed
> with cut-off or PME, so I started reading the section 4.9.1 in the gromacs
> (version 5.1.2) manual to check the involved formulas. Then, I did some
> tests using a model system composed by 2 argon atoms and computing the
> Lennard-Jones (LJ) contribution applying cut-off (without shift the vdw
> potential, ie., using vdwtype=cut-off and vdw-modifier=none in the mdp
> file) with and without dispersion correction (DispCorr=Ener and
> DispCorr=no). After, I computed by hand the dispersion contribution to the
> potential energy for this system. Finally, the values for the dispersion
> contribution obtained from gromacs and by hand were equal.
>
> Next, I tried a second test with the same model and same conditions but
> using PME instead cut-off to treat the vdw interactions with and without
> the dispersion correction. In this second test, the dispersion contribution
>  computed by gromacs was 0. I expected this result because this correction
> is suitable when vdw interactions are computed with cut-off and the radial
> distribution function outside the cutoff is assumed equal to 1. Thus, I
> though it looks incompatible to use dispersion correction (DispCorr=Ener)
> and PME for computing vdw interactions.
>
> However, using a real system as a protein or ligand in water and applying
> PME and the dispersion correction for vdw interactions, gromacs is
> computing a dispersion correction contribution, which is unexpected
> according to the previous tests done before. For this system, gromacs
> prints in the log file the average dispersion constant (<C6>) which I could
> not reproduce manually following equation 4.169 in the gromacs (v. 5.1.2)
> manual. I don't know how gromacs is computing this <C6> value for this
> system with PME.
>
> Finally, My questions are:
> 1. How does gromacs compute the dispersion correction when vdw interactions
> are computed with PME?
The result for the dispersion correction with LJPME are zero when you 
use a geometric combination rule, since the LJPME is exact in principle.
If you use the arithmetic combination rule (e.g. Charmm FF) and LJPME is 
told to use the geometric combiation rule (for efficiency) the 
dispersion correction estimates the difference in dispersion between the 
two. Usually this number is very small.

I suggest you read these two papers:
- Lennard-Jones Lattice Summation in Bilayer Simulations Has Critical
Effects on Surface Tension and Lipid Properties
Christian L. Wennberg, Teemu Murtola, Berk Hess, and Erik Lindahl  J. 
Chem. Theory Comput. 2013, 9, 3527−3537

- Nina M. Fischer, Paul J. van Maaren, Jonas C. Ditz, Ahmet Yildirim and 
David van der Spoel: Properties of Organic Liquids when Simulated with 
Long-Range Lennard-Jones interactions J. Chem. Theory Comput. 11 pp. 
2938-2944 (2015)

> 2. Is it right to apply this correction when vdw interactions is computed
> with PME? if the answer is not, it would be nice if gromacs prints a
> warning message indicating this incompatibility when both options are used.
>
More info would be good indeed.
Maybe you can file a documentation request on http://redmine.gromacs.org
>
> Thank you for your help
>
> Joel Montalvo Acosta
> PhD student at University of Strasbourg
>


-- 
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