[gmx-users] Positive and negative ions move in the same direction under a constant electric field?

林揚善 d93543002 at ntu.edu.tw
Mon May 7 09:57:03 CEST 2007

Dear all:

    In order to make sure that my procedure for applying electric field using Gromacs is correct, I applied electric fields to sodium chloride solvent (SPC216.pdb with NA+ and CL-) in the x direction. After 2 ns running, the trajectories of ions and water molecules showed that both types of ions moved in the same direction, which is obviously wrong because Na+ and Cl- are not discriminated by the electric field. I tried several electric field strengths, ranged from 1 to 0.0001, and the observed phenomena are just the same. 

    In order to delineate the problem, I changed the charge term of NA+ into zero while keeping CL- as -1, in the ions.itp file, and I found that both ions will then move in the opposite direction of the NA+=1 CL-= -1 configuration. 


In summary, for all the cases I tried, Na+ and Cl- moved in the same direction.  

    The essential settings for .mdp file are listed below.


Comm_mode   =    None

Coulombtype   =    PME

Optimize_fft    =    yes

tcoupl             =    berendsen

tc_grps           =    SOL     NA+    CL-

tau_t               =    0.1      0.1      0.1

ref_t               =    300      300     300

constraints      =    none

E_x                =      1       0.1     2.0



Can anyone give me some clue on this? I’ll appreciate for any hints on this question.
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