System

class openmm.openmm.System(*args)

This class represents a molecular system. The definition of a System involves four elements:

  • The set of particles in the system

  • The forces acting on them

  • Pairs of particles whose separation should be constrained to a fixed value

  • For periodic systems, the dimensions of the periodic box

The particles and constraints are defined directly by the System object, while forces are defined by objects that extend the Force class. After creating a System, call addParticle() once for each particle, addConstraint() for each constraint, and addForce() for each Force.

In addition, particles may be designated as “virtual sites”. These are particles whose positions are computed automatically based on the positions of other particles. To define a virtual site, call setVirtualSite(), passing in a VirtualSite object that defines the rules for computing its position.

__init__(self) → System
__init__(self, other) → System

Create a new System.

Methods

__init__(-> System)

Create a new System.

addConstraint(self, particle1, particle2, …)

Add a constraint to the System.

addForce(self, force)

Add a Force to the System.

addParticle(self, mass)

Add a particle to the System.

getConstraintParameters(self, index)

Get the parameters defining a distance constraint.

getDefaultPeriodicBoxVectors(self)

Get the default values of the vectors defining the axes of the periodic box (measured in nm).

getForce(-> Force)

Get a writable reference to one of the Forces in this System.

getForces()

Get the list of Forces in this System

getNumConstraints(self)

Get the number of distance constraints in this System.

getNumForces(self)

Get the number of Force objects that have been added to the System.

getNumParticles(self)

Get the number of particles in this System.

getParticleMass(self, index)

Get the mass (in atomic mass units) of a particle.

getVirtualSite(self, index)

Get VirtualSite object for a particle.

isVirtualSite(self, index)

Get whether a particle is a VirtualSite.

removeConstraint(self, index)

Remove a constraint from the System.

removeForce(self, index)

Remove a Force from the System.

setConstraintParameters(self, index, …)

Set the parameters defining a distance constraint.

setDefaultPeriodicBoxVectors(self, a, b, c)

Set the default values of the vectors defining the axes of the periodic box (measured in nm).

setParticleMass(self, index, mass)

Set the mass (in atomic mass units) of a particle.

setVirtualSite(self, index, virtualSite)

Set a particle to be a virtual site.

usesPeriodicBoundaryConditions(self)

Returns whether or not any forces in this System use periodic boundaries.

Attributes

thisown

The membership flag
property thisown

The membership flag

getNumParticles(self) → int

Get the number of particles in this System.

addParticle(self, mass) → int

Add a particle to the System. If the mass is 0, Integrators will ignore the particle and not modify its position or velocity. This is most often used for virtual sites, but can also be used as a way to prevent a particle from moving.

Parameters

mass (double) – the mass of the particle (in atomic mass units)

Returns

the index of the particle that was added

Return type

int

getParticleMass(self, index) → double

Get the mass (in atomic mass units) of a particle. If the mass is 0, Integrators will ignore the particle and not modify its position or velocity. This is most often used for virtual sites, but can also be used as a way to prevent a particle from moving.

Parameters

index (int) – the index of the particle for which to get the mass

setParticleMass(self, index, mass)

Set the mass (in atomic mass units) of a particle. If the mass is 0, Integrators will ignore the particle and not modify its position or velocity. This is most often used for virtual sites, but can also be used as a way to prevent a particle from moving.

Parameters

  • index (int) – the index of the particle for which to set the mass

  • mass (double) – the mass of the particle

setVirtualSite(self, index, virtualSite)

Set a particle to be a virtual site. The VirtualSite object should have been created on the heap with the “new” operator. The System takes over ownership of it, and deletes it when the System itself is deleted.

Parameters

  • index (int) – the index of the particle that should be treated as a virtual site

  • virtualSite (VirtualSite *) – a pointer to the VirtualSite object describing it

isVirtualSite(self, index) → bool

Get whether a particle is a VirtualSite.

Parameters

index (int) – the index of the particle to check

getVirtualSite(self, index) → VirtualSite

Get VirtualSite object for a particle. If the particle is not a virtual site, this throws an exception.

Parameters

index (int) – the index of the particle to get

getNumConstraints(self) → int

Get the number of distance constraints in this System.

addConstraint(self, particle1, particle2, distance) → int

Add a constraint to the System. Particles whose mass is 0 cannot participate in constraints.

Parameters

  • particle1 (int) – the index of the first particle involved in the constraint

  • particle2 (int) – the index of the second particle involved in the constraint

  • distance (double) – the required distance between the two particles, measured in nm

Returns

the index of the constraint that was added

Return type

int

getConstraintParameters(self, index)

Get the parameters defining a distance constraint.

Parameters

index (int) – the index of the constraint for which to get parameters

Returns

  • particle1 (int) – the index of the first particle involved in the constraint

  • particle2 (int) – the index of the second particle involved in the constraint

  • distance (double) – the required distance between the two particles, measured in nm

setConstraintParameters(self, index, particle1, particle2, distance)

Set the parameters defining a distance constraint. Particles whose mass is 0 cannot participate in constraints.

Parameters

  • index (int) – the index of the constraint for which to set parameters

  • particle1 (int) – the index of the first particle involved in the constraint

  • particle2 (int) – the index of the second particle involved in the constraint

  • distance (double) – the required distance between the two particles, measured in nm

removeConstraint(self, index)

Remove a constraint from the System.

Parameters

index (int) – the index of the constraint to remove

addForce(self, force) → int

Add a Force to the System. The Force should have been created on the heap with the “new” operator. The System takes over ownership of it, and deletes the Force when the System itself is deleted.

Parameters

force (Force *) – a pointer to the Force object to be added

Returns

the index within the System of the Force that was added

Return type

int

getNumForces(self) → int

Get the number of Force objects that have been added to the System.

getForce(self, index) → Force
getForce(self, index) → Force

Get a writable reference to one of the Forces in this System.

Parameters

index (int) – the index of the Force to get

removeForce(self, index)

Remove a Force from the System. The memory associated with the removed Force object is deleted.

Parameters

index (int) – the index of the Force to remove

getDefaultPeriodicBoxVectors(self)

Get the default values of the vectors defining the axes of the periodic box (measured in nm). Any newly created Context will have its box vectors set to these. They will affect any Force added to the System that uses periodic boundary conditions.

Returns

  • a (Vec3) – the vector defining the first edge of the periodic box

  • b (Vec3) – the vector defining the second edge of the periodic box

  • c (Vec3) – the vector defining the third edge of the periodic box

setDefaultPeriodicBoxVectors(self, a, b, c)

Set the default values of the vectors defining the axes of the periodic box (measured in nm). Any newly created Context will have its box vectors set to these. They will affect any Force added to the System that uses periodic boundary conditions.

Triclinic boxes are supported, but the vectors must satisfy certain requirements. In particular, a must point in the x direction, b must point “mostly” in the y direction, and c must point “mostly” in the z direction. See the documentation for details.

Parameters

  • a (Vec3) – the vector defining the first edge of the periodic box

  • b (Vec3) – the vector defining the second edge of the periodic box

  • c (Vec3) – the vector defining the third edge of the periodic box

usesPeriodicBoundaryConditions(self) → bool

Returns whether or not any forces in this System use periodic boundaries.

If a force in this System does not implement usesPeriodicBoundaryConditions a OpenMM::OpenMMException is thrown

Returns

true if at least one force uses PBC and false otherwise

Return type

bool

getForces()

Get the list of Forces in this System

__copy__(self) → System