# fix_modify AtC set reference_potential_energy command

## Syntax

    fix_modify <AtC fixID> set reference_potential_energy [<value|filename>]

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   set reference_potential_energy = name of the AtC sub-command
-   value = optional user specified zero point for PE in native LAMMPS
    energy units
-   filename = optional user specified string for file of nodal PE
    values to be read-in

## Examples

``` LAMMPS
fix_modify AtC set reference_potential_energy
fix_modify AtC set reference_potential_energy -0.05
fix_modify AtC set reference_potential_energy myPEvalues
```

## Description

Used to set various quantities for the post-processing algorithms. It
sets the zero point for the potential energy density using the value
provided for all nodes, or from the current configuration of the lattice
if no value is provided, or values provided within the specified
filename.

## Restrictions

Must be used with [fix atc hardy](fix_atc) or [fix atc field](fix_atc).

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)

## Default

Defaults to the LAMMPS zero point i.e. isolated atoms.