[gmx-users] Unreasonably high pressure values

[Kamps, M.]

Dear GMX users,

I am struggling with understanding my systems behaviour.
I have a semi-isotropic system where my bottom and upper plane are
composed of gold FCC plates. In between is a fluid, which during
equilibration is stationary (no acceleration). I equilibrate the
system with a Berendsen barostat, as the literature suggests. The
compressibility is that of the fluid used (Hexadecane C16H34), while
the reference pressure is 1 bar. See the below section of my systems
MDP:

; Pressure coupling is on
pcoupl                    = berendsen        ; Pressure coupling on
pcoupltype                = semiisotropic            ; uniform scaling
of box vectors
tau_p                    = 2.0                ; time constant, in ps
ref_p                    = 1.0     1.0            ; reference pressure, in bar
compressibility             = 0      8.6e-5            ; isothermal
compressibility, bar^-1

When I equilibrate the system, everything appears to be normal. The
volume shrinks a little to compress the fluid, which leads to a
correct density (correct as in corresponding with known values of the
real-life density). The fluid behaves as expected and everything looks
good.

I then want to use the system to accelerate the fluid, however that
blows up the system. Blowing up as in; fluctuations in the box size,
shrink, expand, rapid shrink, rapid expand, extreme shrink, blowing
up.

Upon inspection, during equilibrating the pressure of my system nicely
converges, however, it converges to -6000 bar, which seems rather
strange.. How does this happen? What am I doing wrong?

I use a 0.5 fs timestep, equilibriate over 400.000 steps (0.2 ns),
300K v-rescale temperature coupling, PME electrostatic calculations.

Kind regards,
Mark