A Volume in OpenPathSampling is something that defines a hypervolume in phase space. This can involve coordinates as well as velocities (or any other snapshot feature, such as box vectors). Since volumes represent a set of points, they can be combined using set-theoretic intersections, unions, etc.

Stable states in path sampling are defined in terms of Volume objects. In addition, OPS defines interfaces in TIS as a Volume.

Basic Volumes


A Volume describes a set of snapshots


Volume which all snapshots can satisfy.


Empty volume: no snapshot can satisfy

Set-Based Volume Combinations

IntersectionVolume(volume1, volume2)

“And” combination (intersection) of two volumes.

UnionVolume(volume1, volume2)

“Or” combination (union) of two volumes.

SymmetricDifferenceVolume(volume1, volume2)

“Xor” combination of two volumes.

RelativeComplementVolume(volume1, volume2)

“Subtraction” combination (relative complement) of two volumes.

VolumeCombination(volume1, volume2, fnc, str_fnc)

Logical combination of volumes.

Collective Variable-Based Volumes

CVDefinedVolume(collectivevariable[, …])

Volume defined by a range of a collective variable collectivevariable.


As with CVDefinedVolume, but for a periodic order parameter.