# fix_modify AtC atom_weight command

## Syntax

    fix_modify <AtC fixID> atom_weight <method> <args>

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   atom_weight = name of the AtC sub-command
-   method = *constant* or *lattice* or *element* or *region* or *group*
    or *read_in*
    -   *constant* \<group-ID\> \<value\>: atoms in specified group are
        assigned the constant value given
    -   *lattice*: volume per atom for specified lattice type (e.g. fcc)
        and parameter
    -   *element*: element volume divided among atoms within element
    -   *region*: volume per atom determined based on the atom count in
        the MD regions and their volumes. Note: meaningful only if atoms
        completely fill all the regions.
    -   *group*: volume per atom determined based on the atom count in a
        group and its volume
    -   *node*: (undocumented)
    -   *node_element*: (undocumented)
    -   *read_in*\<filename\>: list of values for atoms are read-in from
        specified file

## Examples

``` LAMMPS
fix_modify AtC atom_weight constant myatoms 11.8
fix_modify AtC atom_weight lattice
fix_modify AtC atom_weight read-in atm_wt_file.txt
```

## Description

Command for assigning the value of atomic weights used for atomic
integration in atom-continuum coupled simulations.

## Restrictions

The use of the lattice option requires a lattice type and parameter is
already specified.

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)

## Default

*lattice*