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大家好，我现在做一个二维材料在离子液体的溶解和结构研究，我通过packmol通过fixed将几层二维材料和液体混合，二维材料位于box的中心位置，初始结构box设置比较大，
然后将二维材料通过freezegrp固定，通过文献给定材料的电荷和LJ参数，进行NVT模拟，然后想通过NPT进行压缩到一定的密度，但是一直不成功，有时候box一直增加，体系crashed，
下面是我的npt输入：
title = NPT equilibration
;define = -DPOSRES ; position restrain the protein
; Run parameters
integrator = md ; leap-frog integrator
nsteps = 2500000 ; 2 * 500000 = 100 ps
dt = 0.001 ; 2 fs
; Output control
nstxout = 5000 ; save coordinates every 0.2 ps
nstvout = 5000 ; save velocities every 0.2 ps
nstenergy = 1000 ; save energies every 0.2 ps
nstlog = 1000 ; update log file every 0.2 ps
; Bond parameters
continuation = no ; Restarting after NVT
constraints = none ; all bonds (even heavy atom-H bonds) constrained
constraint_algorithm = LINCS
freezegrps = MXE
freezedim = Y Y Y
; Neighborsearching
ns_type = grid ; search neighboring grid cells
nstlist = 10 ; 10 fs
rlist = 1 ; short-range neighborlist cutoff (in nm)
rcoulomb = 1 ; short-range electrostatic cutoff (in nm)
rvdw = 1 ; 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
vdwtype = cutoff
; Temperature coupling is on
tcoupl = nose-hoover ; modified Berendsen thermostat
tc-grps = system ; two coupling groups – more accurate
tau_t = 0.2 ; time constant, in ps
ref_t = 300 ; reference tempure, one for each group, in K; Pressure coupling is on
pcoupl = Berendsen ; Pressure coupling on in NPT
pcoupltype = isotropic ; uniform scaling of box vectors
tau_p = 0.5 ; time constant, in ps
ref_p = 1.0 ; reference pressure, in bar
compressibility = 4.5e-5 ; isothermal compressibility of water, bar^-1
refcoord_scaling = all
; Periodic boundary conditions
pbc = xyz ; 3-D PBC
; Dispersion correction
DispCorr = EnerPres ; account for cut-off vdW scheme
; Velocity generation
gen_vel = yes ; assign velocities from Maxwell distribution
gen_temp = 300 ; temperature for Maxwell distribution
gen_seed = -1 ; generate a random seed不知道怎么回事，大家给点建议。
好像网上说freezegrp不能和NPT一起用，但是我如何构建初始结构呢？达到一定的密度。
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