[gmx-developers] triggering GMX_NB_GENERIC produces wrong interactions

[Igor Leontyev]

Dear gmx-developers,
I would like to test non-standard nonbonded interaction which can not be
implemented simply defining tables. According to posts on gmx-developers 
board, the simplest way to do this  is to modify  "gmx_nb_generic_kernel" 
subroutine. To make use the routine the environment variable 
GMX_NB_GENERIC should be set up. The energy comparison obtained  with "set
GMX_NB_GENERIC=0"  and without, however, produces different energies. What 
can be done?

The system is equilibrated box of 216 SPC/E water molecules.  Initial
temperature, LJ (SR) and Coul. recip. energy terms are the same, while 
Coulomb (SR)  term differs essentially. See bellow output of step 0.

openmpi/mpirun -np2 mdrun -s water216.tpr
======================================================
There are: 648 Atoms
Charge group distribution at step 0: 112 104
Grid: 4 x 3 x 3 cells

Constraining the starting coordinates (step 0)

Constraining the coordinates at t0-dt (step 0)
RMS relative constraint deviation after constraining: 0.00e+00
Initial temperature: 295.174 K

Started mdrun on node 0 Mon May 10 22:43:13 2010

           Step           Time         Lambda
              0        0.00000        0.00000

   Energies (kJ/mol)
        LJ (SR)   Coulomb (SR)   Coul. recip.      Potential    Kinetic En.
    1.73768e+03   -1.04948e+04   -1.13345e+03   -9.89062e+03    1.58418e+03
   Total Energy    Temperature Pressure (bar)
   -8.30643e+03    2.94714e+02   -9.93848e+02

======================================================



export GMX_NB_GENERIC=0
openmpi/mpirun -np2 mdrun -s water216.tpr
======================================================
There are: 648 Atoms
Max number of connections per atom is 2
Total number of connections is 864
Max number of graph edges per atom is 2
Total number of graph edges is 864

Constraining the starting coordinates (step 0)

Constraining the coordinates at t0-dt (step 0)
RMS relative constraint deviation after constraining: 0.00e+00
Initial temperature: 295.174 K

Started mdrun on node 0 Mon May 10 22:25:39 2010

           Step           Time         Lambda
              0        0.00000        0.00000

Grid: 3 x 3 x 3 cells
   Energies (kJ/mol)
        LJ (SR)   Coulomb (SR)   Coul. recip.      Potential    Kinetic En.
    1.73768e+03    4.18426e+04   -1.13345e+03    4.24468e+04    1.58623e+03
   Total Energy    Temperature Pressure (bar)
    4.40330e+04    2.95095e+02    2.00676e+05
======================================================

Igor