# [lammps-users] compressibility

Hi,

As we know, there is a parameter compressibility when using Pressure coupling command in Gromacs.

Is there a similar parameter in LAMMPS? How can I map this parameter to that of LAMMPS?

GROMACS note:
Pressure coupling

#### pcoupl:
no
No pressure coupling. This means a fixed box size.
berendsen
Exponential relaxation pressure coupling with time constant tau_p [ps]. The box is scaled every timestep. It has been argued that this does not yield a correct thermodynamic ensemble, but it is the most efficient way to scale a box at the beginning of a run.
Parrinello-Rahman
Extended-ensemble pressure coupling where the box vectors are subject to an equation of motion. The equation of motion for the atoms is coupled to this. No instantaneous scaling takes place. As for Nose-Hoover temperature coupling the time constant tau_p [ps] is the period of pressure fluctuations at equilibrium. This is probably a better method when you want to apply pressure scaling during data collection, but beware that you can get very large oscillations if you are starting from a different pressure.
#### pcoupltype:
isotropic
Isotropic pressure coupling with time constant tau_p [ps]. The compressibility and reference pressure are set with compressibility [bar-1] and ref_p [bar], one value is needed.
semiisotropic
Pressure coupling which is isotropic in the x and y direction, but different in the z direction. This can be useful for membrane simulations. 2 values are needed for x/y and z directions respectively.
anisotropic
Idem, but 6 values are needed for xx, yy, zz, xy/yx, xz/zx and yz/zy components respectively. When the off-diagonal compressibilities are set to zero, a rectangular box will stay rectangular. Beware that anisotropic scaling can lead to extreme deformation of the simulation box.
surface-tension
Surface tension coupling for surfaces parallel to the xy-plane. Uses normal pressure coupling for the z-direction, while the surface tension is coupled to the x/y dimensions of the box. The first ref_p value is the reference surface tension times the number of surfaces [bar nm], the second value is the reference z-pressure [bar]. The two compressibility [bar-1] values are the compressibility in the x/y and z direction respectively. The value for the z-compressibility should be reasonably accurate since it influences the convergence of the surface-tension, it can also be set to zero to have a box with constant height.
#### tau_p: (1) [ps]
time constant for coupling
#### compressibility: [bar-1]
compressibility (NOTE: this is now really in bar-1) For water at 1 atm and 300 K the compressibility is 4.5e-5 [bar-1].
#### ref_p: [bar]
reference pressure for coupling
#### refcoord_scaling:
no
The reference coordinates for position restraints are not modified. Note that with this option the virial and pressure will depend on the absolute positions of the reference coordinates.
all
The reference coordinates are scaled with the scaling matrix of the pressure coupling.
com
Scale the center of mass of the reference coordinates with the scaling matrix of the pressure coupling. The vectors of each reference coordinate to the center of mass are not scaled. Only one COM is used, even when there are multiple molecules with position restraints. For calculating the COM of the reference coordinates in the starting configuration, periodic boundary conditions are not taken into account.

I think in LAMMPS this is part of the damping (time) constant that
is part of the fix npt command.

Steve