ATC: fix_modify AtC material

LAMMPS Website LAMMPS Manual USER-AtC Manual Commands

### fix_modify AtC material

#### syntax

fix_modify AtC material [elementset_name] [material_id]

#### examples

`fix_modify AtC material gap_region 2`

#### description

Sets the material model in elementset_name to be of type material_id.

#### restrictions

The element set must already be created and the material must be specified in the material file given the the atc fix on construction

#### related

#### default

All elements default to the first material in the material file.