ISOSTRES

NAME
ISOSTRESS - Run MD simulations at constant pressure with an isotropic volume variable

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

DESCRIPTION
This command allows to run simulations and minimizations at a given pressure with isotropic volume changes. If the command is used alone ORAC runs simulations in the NPH ensemble. Simulations in the NPT ensemble can instead be carried out if ISOSTRESS 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$^{-1}$.[80] The system compressibility in MPa$^{-1}$ is read in as input to keyword compr. According to the relation given in Ref. [80] compressibility and frequency should be consistent. If compr is not specified the default value is used.

EXAMPLES
ISOSTRESS PRESS-EXT 0.1 BARO-MASS 10.0 COMPR 5.3e-4
Run a simulation at pressure 0.1 MPA (atmospheric pressure) with a barostat mass corresponding to 10.0 cm$^{-1}$. The compressibility is set to 5.3 $\times$ 10$^{-4}$ MPa$^{-1}$.

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

WARNINGS

1

ORAC can carry out constant pressure runs with isotropic volume changes only for orthogonal cells.

2

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)