[gmx-users] Problems with simulations of a triclinic crystal lattice with Parrinello-Rahman

[sa]

Hello everybody,

I am trying to simulate a triclinic cristal lattice (4x4x2) with gromacs
(4.5.5) in constant pressure with the Parrinello-Rahman (PR) barostat. the
crystal were previously equilibrated in the NPT ensemble during 1ns at 200K
with berendsen thermostat/barostat
. The crystal initail parameters are (28.736   32.292   34.142  91.04
 93.50 110.06 P 1)

I used the following parameters
.....
Tcoupl          = v-rescale
tc-grps         = BCL BEN
tau-t           = 0.1 0.1
ref-t           = 200 200
; Pressure coupling is on
Pcoupl          = Parrinello-Rahman
pcoupltype      = isotropic
tau_p           = 5.0
compressibility = 4.5e-5
ref_p           = 1.0135
gen_vel         = no
DispCorr        = EnerPres
.....
With these parameters, the system explodes.

I am confused here because if I switch to the berendsen barostat with the
following parameters I have no problems to do a simulation of 1ns and the
crystal lattice parameters remains stable (see below)

..... isotropic
Tcoupl          = v-rescale
tc-grps         = BCL BEN
tau-t           = 0.1 0.1
ref-t           = 200 200
; Pressure coupling is on
Pcoupl          = berendsen
pcoupltype      = isotropic
tau_p           = 2.0
compressibility = 4.5e-5
ref_p           = 1.0135
gen_vel         = no
DispCorr        = EnerPres
.....

Energy                      Average   Err.Est.       RMSD  Tot-Drift
-------------------------------------------------------------------------------
Temperature                 199.576      0.023    3.56331  0.0876693  (K)
Pressure                     1.0242       0.25    971.331  -0.346972  (bar)
Box-X                       2.91464    5.7e-05 0.00129697 6.17829e-05  (nm)
Box-Y                       3.07662      6e-05 0.00136905 6.52159e-05  (nm)
Box-Z                        3.4535    6.7e-05 0.00153675 7.3165e-05  (nm)
Volume                      30.9685     0.0018  0.0413433 0.00197128  (nm^3)
T-BCL                       199.516       0.02    3.83149  0.0860062  (K)
T-BEN                       199.981      0.048    9.94395  0.0989034  (K)

CRYST1   29.137   32.742   34.618  91.04  93.50 110.06 P 1

Did I miss something important here about PR and crystals?

SA