[gmx-users] temperature coupling in NVT

[Peter Friedel]

hello to all,
i have a little problem with simulation of a special arrangement: i put 8 preoptimized and to 300 K temperated polystyrene and 512 water molecules into a big box and tried to simulate the behavior of this ensemble as NVT ensemble. the temperature was set to be isotherm with 300 K. but, during and after a preparation run of 500 steps the temperature was cooled down to about 180 K. a further run with the same arrangement and longer simulation time showed the same behavior. i include my mdp-file:

title               =  water_N31-6__-PS_008 position restraining
cpp                 =  /lib/cpp
include             =  -I../../forcefield
define              =  -DPOSRES -DSOLVENTBOX -DFFDREI
constraints         =  none
integrator          =  md
dt                  =  0.0005	; ps !
nsteps              =  500	; total 1.0 ps.
nstcomm             =  1
nstxout             =  10
nstvout             =  1000
nstfout             =  0
nstlog              =  10
nstenergy           =  10
nstlist             =  10
ns_type             =  grid
epsilon_r           =  1.500
coulombtype         =  PME
rlist               =  0.9
rvdw                =  0.9
rcoulomb            =  0.9
fourierspacing      =  0.12
pme_order           =  4
ewald_rtol          =  1e-5
DispCorr            =  EnerPres
xtc_grps            =   wate PS_0 
energygrps          =   wate PS_0  
; Berendsen temperature coupling is on in two groups
Tcoupl              =  berendsen
tau_t               =   0.1 0.1  
tc-grps             =   wate PS_0  
ref_t               =   300 300 
; Pressure coupling is not on
Pcoupl              =  no
Pcoupltype          =  Isotropic
tau_p               =  0.5
compressibility     =  1.0e-5
ref_p               =  1.0
; Generate velocites is on at 300 K.
gen_vel             =  yes
gen_temp            =  293.15
gen_seed            =  173529

additionally, i have to say, that the simulations of water alone and the polystyrene alone showed normal behavior, that means, a normal coupling to the external bath with achieving the 300 K, as it was aimed to.
could anybody help me? what do i do not correctly? 
thank you in advance, peter

Dr. Peter Friedel
Hohe Str. 6, 01069 Dresden
Institut für Polymerforschung Dresden e.V.
Tel.: 0351-4658289
Fax: 0351-4658565
email: friedel at ipfdd.de