Skip to content

OFF-EP 0005 — Allow different electrostatics methods to be used on period and non-periodic systems

Status: Accepted

Authors: Matt Thompson and John Chodera, Jeffrey Wagner, Simon Boothroyd

Stakeholders: Simon Boothroyd, Jeffrey Wagner, David Mobley, John Chodera

Acceptance criteria: Unanimity

Created: 2020-03-15

Discussion: Issue #29

Implementation: openff-standards

Abstract

This change refines the way the <Electrostatics> tag defines behavior in periodic (i.e., condensed-phase) and non-periodic (i.e., gas phase) systems to clarify the intended true electrostatics models in each case. Implementations are permitted to make approximations to these specified models---e.g. Particle-Mesh Ewald (PME)--- as a controlled approximation to Ewald provided the approximation accuracy is controlled.

Motivation and Scope

In version 0.3 of the <Electrostatics> tag, the default electrostatics method is method="PME", with reaction-field also a permitted choice.

These definitions present several issues that this OFF-EP attempts to solve:

  • PME is intended to be a permissible approximation to the true electrostatics model, Ewald
  • The boundary conditions (e.g. dielectric at infinity) for the Ewald sum are not specified
  • The Ewald method is only intended for periodic systems; unmodified vacuum electrostatics are intended for non-periodic systems
  • The treatment of intramolecular electrostatics exceptions is unspecified
  • The choice reaction-field does not uniquely specify the functional form for the true reaction-field model intended; many variants are available
  • The solvent dielectric constant was not specified.
  • Cutoffs were not specified
  • The physical constants used to compute the potential were unspecified

To solve these issues, this OFF-EP proposes:

  • The method attribute is replaced with periodic_potential in analogy to other parameters that use the potential term to specify the functional form or a common choice
  • The periodic_potential attribute defaults to Ewald3D-ConductingBoundary as a valid keyword that states that the Ewald periodic sum with conducting boundary conditions should be the true potential used for periodic systems
  • PME (and other methods) are permissible approximations to Ewald as long as they are controlled.
  • For reaction field or other methods, the periodic_potential can specify the exact functional form used for the periodic potential or a keyword denoting a common choice, along with the optional cutoff and solvent_dielectric attributes
  • The nonperiodic_potential attribute defaults to Coulomb indicating the Coulomb potential is to be used in non-periodic systems, though other functional forms are accepted.
  • The exception_potential attribute defaults to Coulomb, indicating the Coulomb potential is to be used for exceptions, though other functional forms are accepted.
  • We explicitly specify which self-consistent physical constants should be used.

Usage and Impact

Since most force fields use some flavor of PME for periodic systems and something similar to nonperiodic_potential="Coulomb" for non-periodic systems, the default attributes for this tag will likely be the most commonly-used. Splitting method out into explicit attributes to specify periodic, nonperiodic, and exception potential energy terms, however, makes it less ambiguous how electrostatics should be handled in each case and decouples the method used in each case.

Users are recommended to consider upgrading from the default attribute values of 0.3 to 0.4 to avoid continuing to use this ambiguity. Implementations may wish to execute this up-conversion automatically (see below).

If backward-compatibility is provided as specified below, users of old force fields will not need to update their force field definitions.

Backward compatibility

Implementations may wish to add up-converters from old versions. An up-converter could convert the following tag header

<Electrostatics version="0.3" method="PME" scale12="0.0" scale13="0.0" scale14="0.833333" scale15="1.0"/>

to a header using version 0.4, which for this case could be

<Electrostatics version="0.4" periodic_potential="Ewald3D-ConductingBoundary" nonperiodic_potential="Coulomb" exception_potential="Coulomb" scale12="0.0" scale13="0.0" scale14="0.833333" scale15="1.0"/>

Concretely, the following conversions should be performed:

0.3 method 0.4 periodic_potential 0.4 nonperiodic_potential 0.4 exception_potential
PME Ewald3D-ConductingBoundary Coulomb Coulomb
reaction-field charge1*charge2/(4*pi*epsilon0)*(1/r + k_rf*r^2 - c_rf); k_rf=(cutoff^(-3))*(solvent_dielectric-1)/(2*solvent_dielectric+1); c_rf=cutoff^(-1)*(3*solvent_dielectric)/(2*solvent_dielectric+1) Coulomb Coulomb
Coulomb Ewald3D-ConductingBoundary Coulomb Coulomb

If the 0.3 section's method does not actually involve the use of a cutoff or switch_width (such as is the case if method="PME"), those values may be set to their defaults in the 0.4 Electrostatics section.

The value of the 0.4 Electrostatics section's solvent_dielectric should be set to none.

Detailed description

In the general SMIRNOFF spec description

A section is added stating that CODATA 2018 physical constants are used in all released SMIRNOFF versions to date. Future OFF-EPs may migrate the specification of which self-consistent physical constants are used to a higher-level attribute.

In the Electrostatics section

The method tag attribute is removed and replaced with periodic_potential, nonperiodic_potential, and exception_potential.

The optional solvent_dielectric tag attribute is added to specify the solvent dielectric used with finite-ranged potentials, defaulting to none.

For periodic_potential:

  • Ewald3D-ConductingBoundary (default) denotes that the Ewald potential with conducting (dielectric 0) boundary conditions are used
  • A function denotes that the specified function should be used, which may make use of cutoff, switch_width, and/or solvent_dielectric terms
  • Future OFF-EPs may add specific keywords for common choices of reaction field electrostatics

For nonperiodic_potential:

  • Coulomb (default) denotes that the standard Coulomb potential should be used with no cutoff or reaction-field attenuation
  • A function denotes that the specified function should be used, which may make use of cutoff, switch_width, and/or solvent_dielectric terms

For exception_potential:

  • Coulomb (default) denotes that the standard Coulomb potential should be used with no cutoff or reaction-field attenuation
  • A function denotes that the specified function should be used, which may make use of cutoff, switch_width, and/or solvent_dielectric terms

Examples

Ewald electrostatics (permitting PME to be used) are used for periodic systems; Coulomb used for non-periodic:

<Electrostatics version="0.4" periodic_cutoff="None" periodic_potential="Ewald3D-ConductingBoundary" nonperiodic_potential="Coulomb" exception_potential="Coulomb" scale12="0.0" scale13="0.0" scale14="0.833333" scale15="1.0"/>

Shifted reaction field electrostatics (e.g. from OpenMM) are used for periodic systems; Coulomb used for non-periodic:

<Electrostatics version="0.4" periodic_potential="charge1*charge2/(4*pi*epsilon0)*(1/r + k_rf*r^2 - c_rf); k_rf=(cutoff^(-3))*(solvent_dielectric-1)/(2*solvent_dielectric+1); c_rf=cutoff^(-1)*(3*solvent_dielectric)/(2*solvent_dielectric+1)" solvent_dielectric="78.5" periodic_cutoff="12*angstroms" nonperiodic_potential="Coulomb" exception_potential="Coulomb" scale12="0.0" scale13="0.0" scale14="0.833333" scale15="1.0"/>

Alternatives

See OFF-EP 0005.

Discussion

This template is based upon the OFF-EP template.

This document is explicitly CC0 1.0 Universal.