[gmx-users] density tip3p

[nishap.patel at utoronto.ca]

I have a box (3x3x3nm) of Tip3p water molecules ~900 and the density  
when I create the box using genbox is 997.177g/l. I did energy  
minimization run and the potential energy did converge smoothly, so I  
did NPT equilibration run of 100ps and I got the density value of  
975g/l. Why does the density decrease after the run? these are the  
parameters that I used :

  RUN CONTROL PARAMETERS
integrator               = md
tinit                    = 0
dt                       = 0.002
nsteps                   = 50000
nstcomm                  = 0
nstxout                  = 0
nstvout                  = 0
nstfout                  = 0
nstlog                   = 100
nstenergy                = 100
nstxtcout                = 0
xtc_precision            = 1000
nstlist                  = 5
ns-type                  = Grid
pbc                      = xyz
rlist                    = 1.1
coulombtype              = pme
rcoulomb                 = 1.1
epsilon-r                = 1
vdw-type                 = switch
rvdw-switch              = 0.8
rvdw                     = 1.0
Tcoupl                   = V-rescale
tc-grps                  = System
tau_t                    = 0.1
ref_t                    = 300
Pcoupl                   = berendsen
tau_p                    = 0.5
compressibility          = 4e-05
ref_p                    = 1.0
constraints              = all-bonds
constraint-algorithm     = Lincs
unconstrained-start      = no
lincs-order              = 4
lincs-iter               = 1
lincs-warnangle          = 30

Am I missing something?
Thanks

Nisha Patel