[gmx-users] why the pressure fluctuated wildly during the simulation?

[杜波]

when i simulate in the NPT , and the parameter are set as follow,

Pcoupl                   = Parrinello-Rahman
pcoupltype               = isotropic
; Time constant (ps), compressibility (1/bar) and reference P (bar)
tau_p                    = 5
compressibility          = 6.5e-5
ref_p                    = 1.0

and the result:
Energy                      Average   Err.Est.       RMSD  Tot-Drift
-------------------------------------------------------------------------------
Total Energy                30582.9        8.1    301.475      34.36  (kJ/mol)
Temperature                     500    2.9e-05    10.6006 -3.11052e-05  (K)
Pressure                    1.68994      0.044    1199.11 -0.0909901  (bar)

we can see the RMSD  of Pressure is so big !!!
why the pressure fluctuated wildly during the simulation?
thanks!!

regards,
Bo Du
Department of Polymer Science and Engineering,
School of Chemical Engineering and technology,
Tianjin University, Weijin Road 92, Nankai District 300072,
Tianjin City P. R. China
Tel/Fax: +86-22-27404303
E-mail: 2008dubo at gmail.com