# fix evaporate command

## Syntax

    fix ID group-ID evaporate N M region-ID seed

-   ID, group-ID are documented in [fix](fix) command

-   evaporate = style name of this fix command

-   N = delete atoms every this many timesteps

-   M = number of atoms to delete each time

-   region-ID = ID of region within which to perform deletions

-   seed = random number seed to use for choosing atoms to delete

-   zero or more keyword/value pairs may be appended

        keyword = *molecule*
          *molecule* value = *no* or *yes*

## Examples

``` LAMMPS
fix 1 solvent evaporate 1000 10 surface 49892
fix 1 solvent evaporate 1000 10 surface 38277 molecule yes
```

## Description

Remove M atoms from the simulation every N steps. This can be used, for
example, to model evaporation of solvent particles or molecules (i.e.
drying) of a system. Every N steps, the number of atoms in the fix group
and within the specified region are counted. M of these are chosen at
random and deleted. If there are less than M eligible particles, then
all of them are deleted.

If the setting for the *molecule* keyword is *no*, then only single
atoms are deleted. In this case, you should ensure you do not delete
only a portion of a molecule (only some of its atoms), or LAMMPS will
soon generate an error when it tries to find those atoms. LAMMPS will
warn you if any of the atoms eligible for deletion have a non-zero
molecule ID, but does not check for this at the time of deletion.

If the setting for the *molecule* keyword is *yes*, then when an atom is
chosen for deletion, the entire molecule it is part of is deleted. The
count of deleted atoms is incremented by the number of atoms in the
molecule, which may make it exceed *M*. If the molecule ID of the chosen
atom is 0, then it is assumed to not be part of a molecule, and just the
single atom is deleted.

As an example, if you wish to delete 10 water molecules every *N* steps,
you should set *M* to 30. If only the water\'s oxygen atoms were in the
fix group, then two hydrogen atoms would be deleted when an oxygen atom
is selected for deletion, whether the hydrogen atoms are inside the
evaporation region or not.

Note that neighbor lists are re-built on timesteps that atoms are
removed. Thus you should not remove atoms too frequently or you will
incur overhead due to the cost of building neighbor lists.

:::: note
::: title
Note
:::

If you are monitoring the temperature of a system where the atom count
is changing due to evaporation, you typically should use the
[compute_modify dynamic yes](compute_modify) command for the temperature
compute you are using.
::::

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to [binary restart
files](restart). None of the [fix_modify](fix_modify) options are
relevant to this fix.

This fix computes a global scalar, which can be accessed by various
[output commands](Howto_output). The scalar is the cumulative number of
deleted atoms. The scalar value calculated by this fix is \"intensive\".

No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

None

## Related commands

[fix deposit](fix_deposit)

## Default

The option defaults are molecule = no.