Introduction

OpenFF Interchange is a Python package developed by the Open Force Field Initiative for storing, manipulating, and converting molecular mechanics data. Most importantly, the package provides the Interchange class, which stores a molecular mechanics system and provides methods to write the system out in numerous formats.

Interchange’s goals

OpenFF Interchange aims to provide a robust API for producing identical, simulation-ready systems for all major molecular mechanics codes with the Open Force Field software stack. Interchange aims to support systems created with the OpenFF Toolkit, which can be converted to Interchange objects by applying a SMIRNOFF force field from the Toolkit or a Foyer force field. The Interchange object can then produce input files for downstream molecular mechanics software suites. At present, it supports GROMACS, LAMMPS, and OpenMM, and support for Amber and CHARMM is planned.

By design, Interchange supports extensive chemical information about the target system. Downstream MM software generally requires only the atoms present in the system and the parameters for their interactions, but Interchange additionally supports chemical information like their bond orders and partial charges. These data are not present in the final output, but allow the abstract chemical system under study to be decoupled from the implementation of a specific mathematical model. This allows Interchange to easily switch between different force fields for the same system, and supports a simple workflow for force field modification.

Converting in the reverse direction is not a goal of the project; Interchange is not intended to provide general conversions between molecular mechanics codes. This is because individual MM codes each represent the same chemical system in different ways, and the information, especially metadata, required to safely and robustly convert is often not present in simulation input files. If arbitrary conversion is required, consider ParmEd.

Units in Interchange

As a best practice, Interchange always associates explicit units with numerical values. Units are tagged using the openff-units package, which provides numerical types associated with commonly used units and methods for ergonomically and safely converting between units. However, the Interchange API accepts values with units defined by the openmm.units or unyt packages, and will automatically convert these values to the appropriate unit to be stored internally. If raw numerical values without units are provided, Interchange assumes these values are in the correct unit. Explicitly defining units helps minimize mistakes and allows the computer to take on the mental load of ensuring the correct units, so we highly recommend it.

Except where otherwise noted, Interchange uses a nm/ps/K/e/Da unit system commonly used in molecular mechanics software. This forms a coherent set of units compatible with SI:

Quantity

Unit

Symbol

Length

nanometre

nm

Time

picosecond

ps

Temperature

Kelvin

K

Electric charge

electron charge

e

Mass

Dalton

Da

Energy1

kilojoule/mol

kJ/mol

1

Derived unit