# fix_modify AtC on_the_fly command

## Syntax

    fix_modify <AtC fixID> on_the_fly <bond|kernel> <on|off>

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   on_the_fly = name of the AtC sub-command
-   *bond* or *kernel* = specifies on-the-fly calculation of *bond* or
    *kernel* matrix elements
-   *on* or *off* = activate or discontinue on-the-fly mode

## Examples

``` LAMMPS
fix_modify AtC on_the_fly bond on
fix_modify AtC on_the_fly kernel
fix_modify AtC on_the_fly kernel off
```

## Description

Overrides normal mode of pre-calculating and storing bond pair-to-node a
nd kernel atom-to-node matrices. If activated, it will calculate
elements of these matrices during repeated calls of field computations
(i.e. \"on-the-fly\") and not store them for future use. The *on* flag
is optional - if omitted, on_the_fly will be activated for the specified
matrix. Can be deactivated using the *off* flag.

## Restrictions

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

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)

## Default

By default, on-the-fly calculation is not active (i.e. off). However,
THE code does a memory allocation check to determine if it can store all
needed bond and kernel matrix elements. If this allocation fails,
on-the-fly will be activated.