SMIRNOFF (SMIRks Native Open Force Field)

The SMIRNOFF specification

The SMIRNOFF specification can be found in the OpenFF standards repository.

SMIRNOFF and the Toolkit

OpenFF releases all its force fields in SMIRNOFF format. SMIRNOFF is a format developed by OpenFF; its specification can be found in our standards repository. SMIRNOFF-format force fields are distributed as XML files with the .offxml extension. Instead of using atom types like traditional force field formats, SMIRNOFF associates parameters directly with chemical groups using SMARTS and SMIRKS, which are extensions of the popular SMILES serialization format for molecules. SMIRNOFF goes to great lengths to ensure reproducibility of results generated from its force fields.

The OpenFF Toolkit is the reference implementation of the SMIRNOFF spec. The toolkit is responsible for reading and writing .offxml files, for facilitating their modification, and for applying them to a molecular system in order to produce an Interchange object. The OpenFF Interchange project then takes over and is responsible for producing input files and data for actual MD software. The toolkit strives to be backwards compatible with old versions of the spec, but owing to the vagaries of the arrow of time cannot be forward compatible. Trying to use an old version of the toolkit to load an .OFFXML file created with a new version of the spec will lead to an error.

A simplified .offxml file for TIP3P water might look like this:

<?xml version="1.0" encoding="utf-8"?>
<SMIRNOFF version="0.3" aromaticity_model="OEAroModel_MDL">
<Author>The Open Force Field Initiative</Author>
<Date>2021-08-16</Date>
<Constraints version="0.3">
<Constraint smirks="[#1:1]-[#8X2H2+0:2]-[#1]" id="c-tip3p-H-O"
distance="0.9572 * angstrom"></Constraint>
<Constraint smirks="[#1:1]-[#8X2H2+0]-[#1:2]" id="c-tip3p-H-O-H"
distance="1.5139006545247014 * angstrom"></Constraint>
</Constraints>
<vdW version="0.3" potential="Lennard-Jones-12-6" combining_rules="Lorentz-Berthelot"
scale12="0.0" scale13="0.0" scale14="0.5" scale15="1.0" cutoff="9.0 * angstrom"
switch_width="1.0 * angstrom" method="cutoff">
<Atom smirks="[#1]-[#8X2H2+0:1]-[#1]" epsilon="0.1521 * mole**-1 * kilocalorie"
id="n-tip3p-O" sigma="3.1507 * angstrom"></Atom>
<Atom smirks="[#1:1]-[#8X2H2+0]-[#1]" epsilon="0 * mole**-1 * kilocalorie"
id="n-tip3p-H" sigma="1 * angstrom"></Atom>
</vdW>
<Electrostatics version="0.3" scale12="0.0" scale13="0.0" scale14="0.8333333333"
scale15="1.0" cutoff="9.0 * angstrom" switch_width="0.0 * angstrom"
method="PME"></Electrostatics>
<LibraryCharges version="0.3">
<LibraryCharge smirks="[#1]-[#8X2H2+0:1]-[#1]" charge1="-0.834 * elementary_charge"
id="q-tip3p-O"></LibraryCharge>
<LibraryCharge smirks="[#1:1]-[#8X2H2+0]-[#1]" charge1="0.417 * elementary_charge"
id="q-tip3p-H"></LibraryCharge>
</LibraryCharges>
</SMIRNOFF>


Note

TIP3P’s geometry is specified entirely by constraints, but SMIRNOFF certainly supports a wide variety of bonded parameters and functional forms.

Note that this format specifies not just the individual parameters, but also their functional forms and units in very explicit terms. This both makes it easy to read and means that the correct implementation of each force is specifically defined, rather than being left up to the MD engine.

The complicated part is that each parameter is specified by a SMIRKS code. These codes are SMARTS codes with an optional numerical index on some atoms given after a colon. This indexing system comes from SMIRKS. Each parameter expects a certain number of indexed atoms, and applies the force accordingly. Unindexed atoms are used to match the chemistry, but forces are not applied to them. SMARTS/SMIRKS codes are less intimidating than they look; [#1] matches any Hydrogen atom (atomic number 1), while [#8X2H2+0] matches an oxygen atom (atomic number 8) with some additional constraints. Dashes represent bonds. So [#1]-[#8X2H2+0:1]-[#1] represents an oxygen atom indexed as 1 connected to two unindexed hydrogen atoms. This system allows individual parameters to be as general or as specific as needed.

Hint

This page is not the SMIRNOFF spec; it has been moved to the standards repository.