AMBER Archive (2003)

Subject: AMBER: problems with pressure control

From: Endres, Robert G. (endresrg_at_ornl.gov)
Date: Tue Dec 30 2003 - 08:38:02 CST

Dear Amber users,
I have some difficulties with constant pressure MD runs. The system is a solvated molecule inside a box with periodic boundary conditions. After equilibration at 300K with constant volume, I wanted to run at constant pressure. When I choose the default target pressure pres0=1.0 [atm] then the VOLUME of the box slightly shrinks during MD using input
 
&cntrl
   ntc=2, ntf=2, iwrap=1,
   cut=10.0, ntb = 2,
   ntpr=50, ntwx=50,
   nstlim = 1000, dt=0.002,
   ntt=1, temp0=300.0, tautp=1.0,
   ntp=1, pres0=1.0, taup=0.2,
   ntx=5, irest=1, nscm=100
 &end

The PRESS variable in output file starts correspondingly negative (-654.0), and converges to zero. However, when I choose pres0=0.0 for gas phase, the box still shrinks although less strongly. I would have expected that the system would expand at zero applied pressure at 300K (?) I also tried a negative target pressure pres0 which surprisingly didn't give an error message and still led to a slight shrinking of the box. Does anyone understand how the pressure control works (in particular at low pressures)?

Many thanks,
  Robert

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