# fix_modify AtC mass_matrix command

## Syntax

    fix_modify <AtC fixID> mass_matrix <fe|md_fe>

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   mass_matrix = name of the AtC sub-command
-   *fe* or *md_fe* = activate/deactivate using the FE mass matrix in
    the MD region

## Examples

``` LAMMPS
fix_modify AtC mass_matrix fe
```

## Description

Determines whether AtC uses the FE mass matrix based on Gaussian
quadrature or based on atomic quadrature in the MD region. This is
useful for fully overlapping simulations to improve efficiency.

## Restrictions

Should not be used unless the FE region is contained within the MD
region, otherwise the method will be unstable and inaccurate.

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)

## Default

*md_fe*