normal water box length increases along one axis

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10 months 3 weeks ago #7630 by cm
i am doing martini cg simulation using system contains two same proteins,water and chloride ion.the simulation is runnig without givving any error(even upto 2 microsecond),but the problem is when i am seeing the trajectory in vmd it is found that with increasing simulation time the length of the water box along one axis increases .at the begining the water box was cubic.
here is my .mdp file

; Updated 15 Jul 2015 by DdJ
; for use with GROMACS 5
; For a thorough comparison of different mdp options in combination with the Martini force field, see:
; D.H. de Jong et al., Martini straight: boosting performance using a shorter cutoff and GPUs, submitted.
title = Martini

; Most simulations are numerically stable with dt=40 fs,
; however better energy conservation is achieved using a
; 20-30 fs timestep.
; Time steps smaller than 20 fs are not required unless specifically stated in the itp file.

integrator = md
dt = 0.02
nsteps = 50000000
nstcomm = 100
comm-grps =

nstxout = 0
nstvout = 0
nstfout = 0
nstlog = 1000
nstenergy = 100
nstxout-compressed = 1000
compressed-x-precision = 100
compressed-x-grps =
energygrps = PROTEIN W ION

; To achieve faster simulations in combination with the Verlet-neighborlist
; scheme, Martini can be simulated with a straight cutoff. In order to
; do so, the cutoff distance is reduced 1.1 nm.
; Neighborlist length should be optimized depending on your hardware setup:
; updating ever 20 steps should be fine for classic systems, while updating
; every 30-40 steps might be better for GPU based systems.
; The Verlet neighborlist scheme will automatically choose a proper neighborlist
; length, based on a energy drift tolerance.
; Coulomb interactions can alternatively be treated using a reaction-field,
; giving slightly better properties.
; Please realize that electrostVatic interactions in the Martini model are
; not considered to be very accurate to begin with, especially as the
; screening in the system is set to be uniform across the system with
; a screening constant of 15. When using PME, please make sure your
; system properties are still reasonable.
; With the polarizable water model, the relative electrostatic screening
; (epsilon_r) should have a value of 2.5, representative of a low-dielectric
; apolar solvent. The polarizable water itself will perform the explicit screening
; in aqueous environment. In this case, the use of PME is more realistic.

cutoff-scheme = Verlet
nstlist = 20
ns_type = grid
pbc = xyz
verlet-buffer-tolerance = 0.005

coulombtype = reaction-field
rcoulomb = 1.1
epsilon_r = 15 ; 2.5 (with polarizable water)
epsilon_rf = 0
vdw_type = cutoff
vdw-modifier = Potential-shift-verlet
rvdw = 1.1

; normal temperature and pressure coupling schemes can be used.
; It is recommended to couple individual groups in your system separately.
; Good temperature control can be achieved with the velocity rescale (V-rescale)
; thermostat using a coupling constant of the order of 1 ps. Even better
; temperature control can be achieved by reducing the temperature coupling
; constant to 0.1 ps, although with such tight coupling (approaching
; the time step) one can no longer speak of a weak-coupling scheme.
; We therefore recommend a coupling time constant of at least 0.5 ps.
; The Berendsen thermostat is less suited since it does not give
; a well described thermodynamic ensemble.
; Pressure can be controlled with the Parrinello-Rahman barostat,
; with a coupling constant in the range 4-8 ps and typical compressibility
; in the order of 10e-4 - 10e-5 bar-1. Note that, for equilibration purposes,
; the Berendsen barostat probably gives better results, as the Parrinello-
; Rahman is prone to oscillating behaviour. For bilayer systems the pressure
; coupling should be done semiisotropic.

tcoupl = v-rescale
tc-grps = PROTEIN W ION
tau_t = 1.0 1.0 1.0
ref_t = 340 340 340
Pcoupl = parrinello-rahman
Pcoupltype = semiisotropic
tau_p = 12.0 ;parrinello-rahman is more stable with larger tau-p, DdJ, 20130422
compressibility = 3e-4 3e-4 3e-4
ref_p = 1.0 1.0 1.0

gen_vel = no
gen_temp = 340
gen_seed = 473529

; for ring systems and stiff bonds constraints are defined
; which are best handled using Lincs.

constraints = none
constraint_algorithm = Lincs
can anyone please suggest me what is the error.
thanking you.

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10 months 3 weeks ago - 10 months 3 weeks ago #7631 by peterkroon
Replied by peterkroon on topic water box length increases along one axis
This is possible because you have a semiisotropic pressure coupling. If this is unwanted (unphysical) behaviour, use an isotropic pressure coupling.

Also, do NOT make separate temperature coupling groups for solvent and ions. See
Last edit: 10 months 3 weeks ago by peterkroon.

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10 months 3 weeks ago #7632 by cm
Yes......It's working now.
Thank you.

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