[gmx-developers] Problem with Switch potential

[Igor Leontyev]

Dear gmx-developers,
Testing with gmx-5.0 different schemes of Long-Range electrostatics for 
NMA liquid I noticed unreasonable behavior with "coulombtype=Switch". 
Electrostatic energies calculated for the same configuration (i.e. at 0 
MD step) with PME and Cut-off coulombtype reasonably agree between each 
other. But once I use coulombtype=Switch, the electrostatic energy jumps 
by the order of magnitude which is suspicious. See the results bellow. 
Any idea what can be wrong ?

Thank you,
Igor



<coulombtype=PME, rcoulomb=1.3>
Coulomb-14        LJ (SR)  Disper. corr. Coulomb(SR)+Recipr. Potential
-1.29147e+04   -3.28852e+03   -1.28735e+02   -3.54258e+03   -7.86555e+03

<coulombtype=Cut-off, rcoulomb=1.3>
Coulomb-14        LJ (SR)  Disper. corr.   Coulomb (SR)      Potential
-1.29147e+04   -3.28852e+03   -1.28735e+02   -3.53416e+03   -7.85714e+03

<coulombtype=Switch, rcoulomb=1.3, rcoulomb-switch=1.2>
Coulomb-14        LJ (SR)  Disper. corr.   Coulomb (SR)      Potential
-1.29147e+04   -3.28852e+03   -1.28735e+02   -5.25858e+04   -5.69088e+04

=============================================================
                 Long-Range block in mdp-file 
============================================================= 

nstlist             =  5			; update pairlist
rlist               =  1.3			; cut-off for ns
cutoff-scheme    =  group;
coulombtype         =  Switch/PME/Cut-off       ;
rcoulomb            =  1.3      		; cut-off for coulomb
rcoulomb_switch     =  1.2      		; switch for coulomb
vdwtype             = Cut-off
rvdw                =  1.3		; cut-off for vdw
rvdw_switch         =  1.0		; where to start switching vdw
dispcorr            = EnerPres   ; LR dispersion corr for En and Pres
;=======================================================================