[gmx-users] External electric field applied to water box

[Nidhin Thomas]

Hello GROMACS users,

I tried to apply a static electric field across a water box. I used  anisotropic pressure coupling. Details of the pressure coupling is given below.

pcoupl                  = Parrinello-Rahman
pcoupltype              = anisotropic
tau_p                   = 5.0
compressibility         = 4.5e-5 4.5e-5 4.5e-5 0 0 0
ref_p                   = 1.0 1.0 1.0 0 0 0
; 
constraints             = h-bonds
constraint_algorithm    = LINCS
continuation            = yes
;
electric-field-y = 0.1 0 0 0

I see that water box size reduced in the direction of application of electric field (Y) and increased in X and Z direction. 

Original Size = (3.90 x 3.90 x 3.90) nm
Final Size     = (4.39 x 2.66 x 5.16) nm

Could someone please help me understand why the water box is shrinking in the direction of electric field ? can someone please let me know how external electric field is actually implemented in GROMACS ?

Thanks a lot for your help!.

Nidhin Thomas
University of Houston