STRESS

 NAME
STRESS – Run MD simulations at constant pressure with a non-isotropic volume changes

 SYNOPSIS
STRESS [PRESS-EXT pext]  [BARO-MASS wpr] [COMPR compressibility]

 DESCRIPTION
This command allows to run simulations and minimizations at a given pressure with non-isotropic volume changes according to the Parrinello-Rahman equation of motion. If the command is used alone ORAC runs simulations in the NPH ensemble. Simulations in the NPT ensemble can instead be carried out if STRESS is used in conjunction with the command THERMOS. The external pressure in MPa is read in by the keyword PRESS-EXT. Also, the keyword BARO-MASS expects the mass of the barostat in cm$^{-1}$ (see ISOSTRESS). The system compressibility in MPa$^{-1}$ is read in as input to keyword COMPR (see ISOSTRESS)

 EXAMPLES

 
&SIMULATION 
   MDSIM 
   TEMPERATURE 300.0 25.0 
   STRESS PRESS-EXT 0.1 BARO-MASS 10.0 COMPR 1.0e-4
&END

Run a simulation in the NHP ensemble at pressure 0.1 MPA (atmospheric pressure) with a barostat mass corresponding to 10.0 cm$^{-1}$. The compressibility is set to 1.0 $\times$ 10$^{-4}$ MPa$^{-1}$. Velocities are initialized and (optionally scaled) according to a temperature of 300 K.

 
&SIMULATION 
   MDSIM 
   TEMPERATURE 300.0 25.0 
   STRESS PRESS-EXT 0.1 BARO-MASS 10.0 COMPR 1.0e-4
   THERMOS 
     ...
   END 
&END

Same as before but with a Nosé thermostat. The simulation is hence in the NPT ensemble with T=300 K.

 DEFAULTS
ORAC uses the water compressibility at 300 K (i.e. 5.3 $\times$ 10$^{-4}$ MPa$^{-1}$) as the default compressibility.

 WARNINGS
Make sure that when simulations at constant pressure are run ORAC has been compiled with the appropriate PRESSURE option in the config.h file (see Chapter 11)