[gmx-users] Positive energy .

[sujithkakkat .]

Dear all,

    I am running simulations for a water-CO2 system using the OPLSAA
forcefield, where I use TIP4P/Ice water model and the EPM2 model for CO2.
After energy minimization I found the value of potential energy to be
positive, and also the maximum force value was high (I requested Fmax <10
). The values are as follows;

Potential Energy    =  1.7600962e+05
Maximum force     =  6.5140656e+01 on atom 1176
Norm of force        =  8.7009411e+00

Earlier when I did a simulation for gaseous CO2 using the same model, the
potential energies were always positive. But I doubt whether the energy can
be positive here.

I want to make sure that I am not using any wrong parameters, before I
proceed. Please comment.




  My MDP file is as given below;

; Run control
integrator = steep
nsteps     = 20000

; EM criteria and other stuff
emtol     = 10
emstep   = 0.01
niter        = 20
nbfgscorr = 10

; Output control
nstlog       = 1
nstenergy = 1

; Neighborsearching and short-range nonbonded interactions
nstlist    = 1
ns_type = grid
pbc       = xyz
rlist       = 1.0

; Electrostatics

coulombtype = PME
rcoulomb      = 1.0

; van der Waals

vdw-type     = switch
rvdw-switch =0.8
rvdw           = 0.9

; Apply long range dispersion corrections for Energy and Pressure

DispCorr = EnerPres

; Spacing for the PME/PPPM FFT grid

fourierspacing = 0.12

; EWALD/PME/PPPM parameters

pme_order         = 6
ewald_rtol          = 1e-06
epsilon_surface  = 0
optimize_fft        = no

; Temperature and pressure coupling are off during EM

tcoupl  = no
pcoupl = no

; Generate velocities to start

gen_vel = no

; options for bonds

constraints = all-bonds

; Type of constraint algorithm

constraint-algorithm = lincs

; Do not constrain the starting configuration

continuation = no

; Highest order in the expansion of the constraint coupling matrix

lincs-order = 12