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

David van der Spoel spoel at xray.bmc.uu.se
Sun Nov 16 14:54:01 CET 2014

On 2014-11-15 19:34, Berk Hess wrote:
> 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.

Thanks. There is more though. I find that the exclusion/self corrections 
are turned off when using Verlet scheme.

The routine ewald_LRcorrection is called (in both 4.6.x and 5.0.x) such 
that the variable calc_excl_corr is set as ir->cutoff_scheme != 
ecutsVERLET. This means in practice that with the Verlet scheme the self 
and exclusion corrections are 0.

Is this intentional? The effect is a large energy difference between 
systems in the two versions (we are trying to get the polarizable drude 
water working).

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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