Forces (Module)

Overview

The Forces module calculates the total atomic forces present in one or more configurations arising from intramolecular and intermolecular interactions between atoms. It is not normally required to be present in a typical simulation - modules such as MD directly call the necessary force functions, for instance - but it can be used to test against reference data in order to ensure consistency with other simulation codes.

Description

Basic Theory

The total forces occurring within a system of $M$ molecules (comprising $N$ atoms) is given by

$$ F_{total} = F_{geometry} + F_{pairpotential} $$

where

$$ F_{geometry} = \sum^M_{m} \left( \sum^{bonds}b F^b{ij} + \sum^{angles}a F^a{ijk} + \sum^{torsions}t F^t{ijkl} + \sum^{impropers}i F^i{ijkl} \right) $$

$$ F_{pairpotential} = \sum^N_{i,j \ge i} F^{LJ}_{ij} + \frac{q_i q_j}{r} $$

While $F_{geometry}$ is calculate analytically using the relevant functional forms of the interactions, $F_{pairpotential}$ always uses a tabulated force for the short-range Lennard-Jones part of the interaction. Depending on how Dissolve is set up, this tabulated potential may also include the forces arising from the associated Coulomb interactions.

$F_{pairpotential}$ is calculated using standard rules for interactions occurring within the same molecule - 1,2 and 1,3 interactions (i.e. those between atoms at either end of defined bonds or angles) are excluded completely, while 1,4 interactions (i.e. between atoms at either ends of torsion interactions) are scaled by a factor of one half.

Interaction Forms

Dissolve implements several functional forms for each of the distinct intra- and interatomic interaction types, and which are detailed in the functional forms section.

Options

Targets

Keyword	Arguments	Default	Description
Configuration	Configuration	–	Required Target configuration on which to operate.

Export

Keyword	Arguments	Default	Description
SaveForces	ForcesFileAndFormat	–	Save calculated energies to the specified file / format

Advanced

Keyword	Arguments	Default	Description
Test	bool	false	Test parallel force routines against basic serial versions and supplied reference values (if provided)
TestAnalytic	bool	false	Use analytic interatomic energies rather than (production) tabulated potentials for tests
TestInter	bool	true	Include interatomic forces in test
TestIntra	bool	true	Include intramolecular forces in test
TestReference	ForcesFileAndFormat	–	Reference forces to test calculated forces against
TestThreshold	double	0.1	Threshold of force (%) at which test comparison will fail