# compute pressure/cylinder command

## Syntax

compute ID group-ID pressure/cylinder zlo zhi Rmax bin_width

• ID, group-ID are documented in compute command

• pressure/cylinder = style name of this compute command

• zlo = minimum z-boundary for cylinder

• zhi = maximum z-boundary for cylinder

• Rmax = maximum radius to perform calculation to

• bin_width = width of radial bins to use for calculation

## Examples

compute 1 all pressure/cylinder -10.0 10.0 15.0 0.25


## Description

Define a computation that calculates the pressure tensor of a system in cylindrical coordinates, as discussed in (Addington). This is useful for systems with a single axis of rotational symmetry, such as cylindrical micelles or carbon nanotubes. The compute splits the system into radial, cylindrical-shell-type bins of width bin_width, centered at x=0,y=0, and calculates the radial (P_rhorho), azimuthal (P_phiphi), and axial (P_zz) components of the configurational pressure tensor. The local density is also calculated for each bin, so that the true pressure can be recovered as P_kin+P_conf=density*k*T+P_conf. The output is a global array with 5 columns; one each for bin radius, local number density, P_rhorho, P_phiphi, and P_zz. The number of rows is governed by the values of Rmax and bin_width. Pressure tensor values are output in pressure units.

## Output info

This compute calculates a global array with 5 columns and Rmax/bin_width rows. The output columns are: R (distance units), number density (inverse volume units), configurational radial pressure (pressure units), configurational azimuthal pressure (pressure units), and configurational axial pressure (pressure units).

The values calculated by this compute are “intensive”. The pressure values will be in pressure units. The number density values will be in inverse volume units.

## Restrictions

This compute currently calculates the pressure tensor contributions for pair styles only (i.e. no bond, angle, dihedral, etc. contributions and in the presence of bonded interactions, the result will be incorrect due to exclusions for special bonds) and requires pair-wise force calculations not available for most many-body pair styles. K-space calculations are also excluded. Note that this pressure compute outputs the configurational terms only; the kinetic contribution is not included and may be calculated from the number density output by P_kin=density*k*T.

This compute is part of the USER-MISC package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.

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