[gmx-developers] Problem with Switch potential

[Igor Leontyev]

I have uploaded my input and output files here 
http://redmine.gromacs.org/issues/1569
just for the case if somebody want to look at this in greater details.

Igor

-------- Original Message --------
Subject: Problem with Switch potential
Date: Tue, 22 Jul 2014 20:36:35 -0700
From: Igor Leontyev <ileontyev at ucdavis.edu>
To: gmx-developers at gromacs.org

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
;=======================================================================