[gmx-developers] DIfference in Coulomb terms 5.0/4.6

[Berk Hess]

Hi,

Is this group vs Verlet cutoff scheme? With the Verlet Cutoff's scheme the subtraction of the reciprocal component for excluded pair has been moved from the reciprocal term to the SR term.
That is not really debugging, since the division is not based on physics and thus there is no bug.
Additionally the automated PME load balancing can change beta, thereby changing the distribution of the total Coulomb energy over the two terms.

Cheers,

Berk

On Nov 15, 2014 7:15 PM, David van der Spoel <spoel at xray.bmc.uu.se> wrote:
>
> We're trying to debug the difference between Coulomb SR/Recip division 
> in 4.6 and 5.0. To simplify things we made a system of 2 SPC molecules. 
> They use the same cut-off, even after mdrun changes if, and the distance 
> between the molecules is such that they are completely within the short 
> range cutoff. Results are below. Any clue what causes the difference? 
>
> gmx-master: 
> Statistics over 1 steps [ 0.0000 through 0.0000 ps ], 5 data sets 
> All statistics are over 1 points 
>
> Energy                      Average   Err.Est.       RMSD  Tot-Drift 
> ------------------------------------------------------------------------------- 
> LJ (SR)                     7.92898         --          0          0 
> (kJ/mol) 
> Disper. corr.            -0.00105424         --          0          0 
> (kJ/mol) 
> Coulomb (SR)               -41.7249         --          0          0 
> (kJ/mol) 
> Coul. recip.                6.32359         --          0          0 
> (kJ/mol) 
> Potential                  -27.4734         --          0          0 
> (kJ/mol) 
>
>
> gmx-4.6.7: 
> Statistics over 1 steps [ 0.0000 through 0.0000 ps ], 5 data sets 
> All statistics are over 1 points 
>
> Energy                      Average   Err.Est.       RMSD  Tot-Drift 
> ------------------------------------------------------------------------------- 
> LJ (SR)                     7.92898         --          0          0 
> (kJ/mol) 
> Disper. corr.            -0.00105424         --          0          0 
> (kJ/mol) 
> Coulomb (SR)               -36.0859         --          0          0 
> (kJ/mol) 
> Coul. recip.               0.684564         --          0          0 
> (kJ/mol) 
> Potential                  -27.4734         --          0          0 
> (kJ/mol) 
>
>
> David. 
> -- 
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