# Potential File Format 5

## Introduction

Since OpenKIM itself is not a common interaction but a library for all kinds of interatomic models the way potfit behaves when OpenKIM support is enabled is a little bit different than with the regular interactions. Before being able to use OpenKIM interactions with *potfit* you have to install the OpenKIM package as well as any models you would like to use. For more on how to do this please see the OpenKIM homepage for detailed instructions. Once this is done you can build a OpenKIM-enabled version of *potfit*.

## Format specification

The potential format `5`

is used for OpenKIM potentials.
All header tokens are supported except for the `#G`

line.

Each parameter of the OpenKIM potential has to be specified as follows:

<param_type> <param_name> <param_value_1> <param_value_1_min> <param_value_1_max> <param_value_2> <param_value_2_min> <param_value_2_max> ... <param_value_n> <param_value_n_min> <param_value_n_max>

<param type> can be either 'KIM_PARAM' for a regular parameter of 'KIM_CUTOFF' for the cutoff parameter of a model. If you don't know if the model publishes the cutoff as an independent parameter just use 'KIM_PARAM' for all parameters.

<param_name> has to be exactly the same string as the model uses in the published parameter list.

Depending on the dimensionality of the parameter, it requires N lines of values, which consist of the initial value, the minimum and maximum allowed values.

Here is an example for the OpenKIM model 'ex_model_Ar_P_LJ' which publishes three parameters, which all have the dimension 1:

#F 5 1 #T ex_model_Ar_P_LJ #C Ar #E KIM_PARAM cutoff 8.150000 8.150000 8.150000 KIM_PARAM epsilon 0.010400 0.010400 0.010400 KIM_PARAM sigma 3.400000 3.400000 3.400000

## Additional features

### Global parameters

*potfit* supports global parameters also for OpenKIM potentials. A detailed explanation can be found on the analytic potentials page.