[gmx-users] Positive PE in minimization step of ionic liquid BMIM PF6

[Abhishek Banerjee]

Hi
    I am new to Gromacs. I am trying to simulate ionic liquids BMIM PF6.
First I made a simulation box with 128 ion pairs with packmol. *Then I did
energy minimization. But I got positive PE*. I also did equilibration with
npt and got reasonable density. *Is it normal for ionic liquids?* I
searched a lot but did not get any answer.

Steepest Descents converged to Fmax < 1000 in 269 steps
Potential Energy  =  1.0977283e+05
Maximum force     =  7.7167487e+02 on atom 3418
Norm of force     =  7.8315048e+01

Here is my .mdp file
integrator    = steep
emtol        = 1000.0
emstep          = 0.02
nsteps        = 50000

; Parameters describing how to find the neighbors of each atom and how to
calculate the interactions
nstlist            = 10
cutoff-scheme = Verlet
ns_type            = grid        ; Method to determine neighbor list
(simple, grid)
coulombtype     = PME        ; Treatment of long range electrostatic
interactions
rcoulomb           = 1.4        ; Short-range electrostatic cut-off
rvdw                  = 1.4        ; Short-range Van der Waals cut-off
pbc                    = xyz         ; Periodic Boundary Conditions (yes/no)

Hope some one will help me to sort out the problem. Thanks for your time.


Dr. Abhishek Banerjee
Young Scientist, MRI-MRS centre
Dept. of Chemistry, IIT Madras
Chennai - 600036, Tamil Nadu, India
Mob. No. 07598373142