[gmx-users] Query about a simple MD with constant electric field

[Ivan Gladich]

Dear Gromacs users

I am doing a very simple MD with a constant electric field of 1 V/nm 
along the positive z-direction in a system containing only one Na+ ion, 
starting from rest.
The ion is not coupled to the thermostat and it is placed in a box of 4 
nm* 4nm * 16 nm, in the X,Y,and Z direction respectively.

  I would expect my ion accelerating along the positive Z-direction with 
constant acceleration.
However my ion  starts to move in the positive  Z-direction but with 
constant velocity for all my 1 ns MD. Do I do something really wrong?

I am using gromacs 5.1.1 in double precision

Thanks for any possible help
Below my inputs
Ivan

###########
conf.gro
###########

Gyas ROwers Mature At Cryogenic Speed
  1
  2183Na      Na 6545   3.014   0.540   8.909
    4.00000   4.00000  16.00000

##########
topol.top
#########

[ defaults ]
; nbfunc        comb-rule       gen-pairs       fudgeLJ fudgeQQ
1               2               yes             0.5     0.8333

[ atomtypes ]
;name  at.num   mass       charge   ptype   sigma   epsilon
Na        11      79.90    1.0000  A    0.385       0.0022

[ moleculetype ]
; molname       nrexcl
Na              1

[ atoms ]
; id    at type         res nr  residu name     at name  cg nr  charge
1       Na              1       Na              Na       1      1.00000

[ system ]
  THE-BEST

[ molecules ]
; Compound        nmols
  Na              1


#########
grompp.mdp
#########

title       = Na+Electric

; Run parameters
integrator  = md        ; leap-frog integrator
nsteps      = 1000000   ;  = 1000ps
dt          = 0.001     ; 1 fs

; Output control
nstxout     = 0       ; save coordinates every 1.0 ps
nstvout     = 1000       ; save velocities every 1.0 ps
nstenergy   = 5000    ; save energies every 1 ps
nstlog      = 5000    ; update log file every 1 ps
nstxtcout   = 1000    ; xtc compressed trajectory output every 1 ps

; Neighborsearching
cutoff-scheme   = group ;Verlet
ns_type         = grid      ; search neighboring grid cells
nstlist         = 1        ; 20 fs, largely irrelevant with Verlet
rlist           = 0.9
rcoulomb        = 0.9       ; short-range electrostatic cutoff (in nm)
rvdw            = 0.9       ; short-range van der Waals cutoff (in nm)

; Electrostatics
coulombtype     = PME       ; Particle Mesh Ewald for long-range 
electrostatics
pme_order       = 4         ; cubic interpolation
fourierspacing  = 0.16      ; grid spacing for FFT

; Temperature coupling
tcoupl      = no ;

; Pressure coupling
pcoupl      = no;

; Periodic boundary conditions
pbc         = xyz       ; 3-D PBC

; Dispersion correction
DispCorr    = no ;

; Velocity generation
gen_vel     = no

;Electric

E-x = 0 0 0
E-y = 0 0 0
E-z = 1 1 0 : 1 V nm-1