[gmx-users] Erik.Please help me! Some question about the NPT equirbrium CCl4 Box!

[PeiQuan Chen]

Dear gmx-users：
  I am in the process of equilibrating a box of CCl4 in NPT!
  But in this case, there must be something I am doing wrong
because I don't get the right density(calculate density>exp 
density). I construct a box that contained 216 molecules,and 
perform Em on that box. Afther that I make a 500ps NVT 
Dynamics on that to get a NVT equibrium box. Above all is 
successful. But When I do a 1500ps NPT dynamics on the box 
of NVT equibrium, It begins wrong with my system. The system
potential decrease with times and the box vector begin to 
decrease too(3.25878-->3.13426). Can anybody give me a hint how
to get the right NPT equibrium box of the system?

That following is the files I use to do the simulation:

ccl4.itp:
;atomtypes the user defined refer to the oplsaa force field
[ atomtypes ]
; name    bond_type    mass      charge    ptype      sigma      epsilon
  opls_001   CT     12.01100     0.248       A    3.80000e-01  2.09340e-01
  opls_002   Cl     35.45300    -0.062       A    3.47000e-01  1.11369e+00

[ moleculetype ]
; Name            nrexcl
  CCl4             3
[ atoms ]
;   nr       type  resnr residue  atom cgnr    charge       mass  
     1   opls_001      1    CTC      C    1     0.248     12.011  
     2   opls_002      1    CTC    Cl1    1    -0.062     35.453  
     3   opls_002      1    CTC    Cl2    1    -0.062     35.453  
     4   opls_002      1    CTC    Cl3    1    -0.062     35.453  
     5   opls_002      1    CTC    Cl4    1    -0.062     35.453

[ bonds ]
;  ai    aj funct          b0        kb  
    1     2     1 	0.17810   205153.2
    1     3     1 	0.17810   205153.2
    1     4     1 	0.17810   205153.2
    1     5     1 	0.17810   205153.2

[ angles ]
;  ai    aj    ak funct          th0       cth
    2     1     3     1 	111.700    653.141
    2     1     4     1 	111.700    653.141
    2     1     5     1 	111.700    653.141
    3     1     4     1 	111.700    653.141
    3     1     5     1 	111.700    653.141
    4     1     5     1 	111.700    653.141

box.top:

;Include forcefield parameters
#include "ffoplsaa.itp"
#include "ccl4.itp"
[ system ]
; Name
Carbon tetracholoride box( 3.2587807nm 3.2587807nm 3.2587807nm )

[ molecules ]
; Compound        #mols
   CCl4            216   

md.mdp:
;	User spoel (236)
;	Wed Nov  3 17:12:44 1993
;	Input file
;

title               =  md  mdp file
cpp                 =  /lib/cpp
define              =  -DFLEX_SPC
constraints         =  none
integrator          =  md
dt                  =  0.001
nsteps              =  1500000

nstcomm             =  1
nstxout             =  500
nstvout             =  1000
nstfout             =  0
nstlog              =  10
nstenergy           =  1000   
nstlist             =  10

ns_type             =  grid
coulombtype         =  PME
rlist               =  0.9
rcoulomb            =  0.9
rvdw                =  0.9
DispCorr            =  EnerPres
fourierspacing      =  0.12
fourier_nx          =  0
fourier_ny          =  0
fourier_nz          =  0
pme_order           =  4
ewald_rtol          =  1e-5
optimize_fft        =  yes
;Berendsen temperature coupling is on in the system.
Tcoupl              =  berendsen
tau_t               =  0.1
tc-grps             =  system
ref_t               =  300
;Pressure coupling is not on
Pcoupl              =  berendsen
pcoupltype          =  isotropic
tau_p               =  0.5
compressibility     =  4.5e-5
ref_p               =  1.0
;Generate velocites is on at 300 K.
gen_vel             =  yes
gen_temp            =  300.0
gen_seed            =  173529

Does it seems something wrong of my files? And does the 
oplsaa force field is not suitabe for CCl4? And how to get 
a equibrium box of CCl4?

Thank you in advance!












Sincerely yours
---- 
PeiQuan Chen
________________________________________________________________________
PeiQuan Chen 	Department of Chemistry, Nankai University
WeiJin Road 94, TianJin, 300071, P.R.China
phone:	(86)2223506728		Mobile Phone: (+86)13920675030
gromacs at 163.com	
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