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# compute pressure/alchemy command

## Syntax

    compute ID group-ID pressure/alchemy fix-ID

- ID, group-ID are documented in compute command

- pressure/alchemy = style name of this compute command

- fix-ID = ID of fix alchemy command

## Examples

    fix trans all alchemy
    compute mixed all pressure/alchemy trans
    thermo_modify press mixed

## Description

Added in version 28Mar2023.

Define a compute style that makes the “mixed” system pressure available for a system that uses the fix alchemy command to transform one topology to another. This can be used in combination with either thermo_modify press or fix_modify press to output and access a pressure consistent with the simulated combined two topology system.

The actual pressure is determined with compute pressure commands that are internally used by fix alchemy for each topology individually and then combined. This command just extracts the information from the fix.

The `examples/PACKAGES/alchemy` folder contains an example input for this command.

## Output info

This compute calculates a global scalar (the pressure) and a global vector of length 6 (the pressure tensor), which can be accessed by indices 1–6. These values can be used by any command that uses global scalar or vector values from a compute as input. See the Howto output page for an overview of LAMMPS output options.

The ordering of values in the symmetric pressure tensor is as follows: \(p_{xx},\) \(p_{yy},\) \(p_{zz},\) \(p_{xy},\) \(p_{xz},\) \(p_{yz}.\)

The scalar and vector values calculated by this compute are “intensive”. The scalar and vector values will be in pressure units.

## Restrictions

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

## Related commands

fix alchemy, compute pressure, thermo_modify, fix_modify

## Default

none