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.
;
; File 'mdout.mdp' was generated
; By user: Yang_Shan (500)
; On host: localhost.localdomain
; At date: Mon Feb 5 10:34:03 2007
;
; VARIOUS PREPROCESSING OPTIONS
title = Yo
; Preprocessor - specify a full path if necessary.
cpp = /usr/bin/cpp
include =
define =
; RUN CONTROL PARAMETERS
integrator = md
; Start time and timestep in ps
tinit = 0
dt = 0.002
nsteps = 1000000
; For exact run continuation or redoing part of a run
init_step = 0
; mode for center of mass motion removal
comm-mode = none
; number of steps for center of mass motion removal
; nstcomm = 1
; group(s) for center of mass motion removal
comm-grps =
; LANGEVIN DYNAMICS OPTIONS
; Friction coefficient (amu/ps) and random seed
bd-fric = 0
ld-seed = 1993
; ENERGY MINIMIZATION OPTIONS
; Force tolerance and initial step-size
emtol = 100
emstep = 0.01
; Max number of iterations in relax_shells
niter = 20
; Step size (ps^2) for minimization of flexible constraints
fcstep = 0
; Frequency of steepest descents steps when doing CG
nstcgsteep = 1000
nbfgscorr = 10
; OUTPUT CONTROL OPTIONS
; Output frequency for coords (x), velocities (v) and forces (f)
nstxout = 400
nstvout = 400
nstfout = 0
; Checkpointing helps you continue after crashes
nstcheckpoint = 1000
; Output frequency for energies to log file and energy file
nstlog = 400
nstenergy = 400
; Output frequency and precision for xtc file
; nstxtcout = 50
; xtc-precision = 1000
; This selects the subset of atoms for the xtc file. You can
; select multiple groups. By default all atoms will be written.
; xtc-grps =
; Selection of energy groups
energygrps = SOL
; NEIGHBORSEARCHING PARAMETERS
; nblist update frequency
nstlist = 5
; ns algorithm (simple or grid)
ns_type = grid
; Periodic boundary conditions: xyz (default), no (vacuum)
; or full (infinite systems only)
pbc = xyz
; nblist cut-off
rlist = 0.9
domain-decomposition = no
; OPTIONS FOR ELECTROSTATICS AND VDW
; Method for doing electrostatics
coulombtype = PME
rcoulomb-switch = 0
rcoulomb = 0.9
; Relative dielectric constant for the medium and the reaction field
epsilon-r = 1
epsilon_rf = 1
; Method for doing Van der Waals
vdw-type = Cut-off
; cut-off lengths
rvdw-switch = 0
rvdw = 0.9
; Apply long range dispersion corrections for Energy and Pressure
DispCorr = EnerPres
; Extension of the potential lookup tables beyond the cut-off
table-extension = 1
; Seperate tables between energy group pairs
energygrp_table =
; Spacing for the PME/PPPM FFT grid
fourierspacing = 0.12
; FFT grid size, when a value is 0 fourierspacing will be used
fourier_nx = 0
fourier_ny = 0
fourier_nz = 0
; EWALD/PME/PPPM parameters
pme_order = 4
ewald_rtol = 1e-05
ewald_geometry = 3d
epsilon_surface = 0
optimize_fft = yes
; GENERALIZED BORN ELECTROSTATICS
; Algorithm for calculating Born radii
gb_algorithm = Still
; Frequency of calculating the Born radii inside rlist
nstgbradii = 1
; Cutoff for Born radii calculation; the contribution from atoms
; between rlist and rgbradii is updated every nstlist steps
rgbradii = 2
; Salt concentration in M for Generalized Born models
gb_saltconc = 0
; IMPLICIT SOLVENT (for use with Generalized Born electrostatics)
implicit_solvent = No
; OPTIONS FOR WEAK COUPLING ALGORITHMS
; Temperature coupling
Tcoupl = berendsen
; Groups to couple separately
tc-grps = SOL NA+ CL-
; Time constant (ps) and reference temperature (K)
tau_t = 0.1 0.1 0.1
ref_t = 300 300 300
; Pressure coupling
Pcoupl = berendsen
Pcoupltype = isotropic
; Time constant (ps), compressibility (1/bar) and reference P (bar)
tau_p = 0.5
compressibility = 4.5e-5
ref_p = 1.0
; Random seed for Andersen thermostat
andersen_seed = 815131
; OPTIONS FOR QMMM calculations
QMMM = no
; Groups treated Quantum Mechanically
QMMM-grps =
; QM method
QMmethod =
; QMMM scheme
; QMMMscheme = normal
; QM basisset
QMbasis =
; QM charge
QMcharge =
; QM multiplicity
QMmult =
; Surface Hopping
SH =
; CAS space options
CASorbitals =
CASelectrons =
SAon =
SAoff =
SAsteps =
; Scale factor for MM charges
; MMChargeScaleFactor = 1
; Optimization of QM subsystem
bOPT =
bTS =
; SIMULATED ANNEALING
; Type of annealing for each temperature group (no/single/periodic)
annealing = no
; Number of time points to use for specifying annealing in each group
annealing_npoints =
; List of times at the annealing points for each group
annealing_time =
; Temp. at each annealing point, for each group.
annealing_temp =
; GENERATE VELOCITIES FOR STARTUP RUN
gen_vel = yes
gen_temp = 300
gen_seed = 1993
; OPTIONS FOR BONDS
constraints = none
; Type of constraint algorithm
constraint-algorithm = Lincs
; Do not constrain the start configuration
unconstrained-start = no
; Use successive overrelaxation to reduce the number of shake iterations
Shake-SOR = no
; Relative tolerance of shake
shake-tol = 1e-04
; Highest order in the expansion of the constraint coupling matrix
lincs-order = 4
; Number of iterations in the final step of LINCS. 1 is fine for
; normal simulations, but use 2 to conserve energy in NVE runs.
; For energy minimization with constraints it should be 4 to 8.
lincs-iter = 1
; Lincs will write a warning to the stderr if in one step a bond
; rotates over more degrees than
lincs-warnangle = 30
; Convert harmonic bonds to morse potentials
morse = no
; ENERGY GROUP EXCLUSIONS
; Pairs of energy groups for which all non-bonded interactions are excluded
energygrp_excl =
; NMR refinement stuff
; Distance restraints type: No, Simple or Ensemble
disre = No
; Force weighting of pairs in one distance restraint: Conservative or Equal
disre-weighting = Conservative
; Use sqrt of the time averaged times the instantaneous violation
disre-mixed = no
disre-fc = 1000
disre-tau = 0
; Output frequency for pair distances to energy file
nstdisreout = 100
; Orientation restraints: No or Yes
orire = no
; Orientation restraints force constant and tau for time averaging
orire-fc = 0
orire-tau = 0
orire-fitgrp =
; Output frequency for trace(SD) and S to energy file
nstorireout = 100
; Dihedral angle restraints: No, Simple or Ensemble
dihre = No
dihre-fc = 1000
dihre-tau = 0
; Output frequency for dihedral values to energy file
nstdihreout = 100
; Free energy control stuff
free-energy = no
init-lambda = 0
delta-lambda = 0
sc-alpha = 0
sc-power = 1
sc-sigma = 0.3
; Non-equilibrium MD stuff
acc-grps =
accelerate =
freezegrps =
freezedim =
cos-acceleration = 0
deform =
; Electric fields
; Format is number of terms (int) and for all terms an amplitude (real)
; and a phase angle (real)
E-x = 1 0.1 1.0
E-xt =
E-y =
E-yt =
E-z =
E-zt =
; User defined thingies
user1-grps =
user2-grps =
userint1 = 0
userint2 = 0
userint3 = 0
userint4 = 0
userreal1 = 0
userreal2 = 0
userreal3 = 0
userreal4 = 0
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