# compute pressure/alchemy command

## Syntax

``` LAMMPS
compute ID group-ID pressure/alchemy fix-ID
```

-   ID, group-ID are documented in [compute](compute) command
-   pressure/alchemy = style name of this compute command
-   fix-ID = ID of [fix alchemy](fix_alchemy) command

## Examples

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

## Description

::: versionadded
28Mar2023
:::

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

The actual pressure is determined with [compute
pressure](compute_pressure) commands that are internally used by [fix
alchemy](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](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](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](Build_package) page for more info.

## Related commands

[fix alchemy](fix_alchemy), [compute pressure](compute_pressure),
[thermo_modify](thermo_modify), [fix_modify](fix_modify)

## Default

none