fix evaporate command


fix ID group-ID evaporate N M region-ID seed
  • ID, group-ID are documented in 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


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


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 insure 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 hydrogens 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.


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 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. None of the fix_modify options are relevant to this fix.

This fix computes a global scalar, which can be accessed by various output commands. 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 command. This fix is not invoked during energy minimization.


This fix is part of the MISC package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.


The option defaults are molecule = no.