# Force¶

class OpenMM::Force

Force objects apply forces to the particles in a System, or alter their behavior in other ways. This is an abstract class. Subclasses define particular forces.

More specifically, a Force object can do any or all of the following:

• Add a contribution to the force on each particle
• Add a contribution to the potential energy of the System
• Modify the positions and velocities of particles at the start of each time step
• Define parameters which are stored in the Context and can be modified by the user
• Change the values of parameters defined by other Force objects at the start of each time step

Forces may be organized into “force groups”. This is used for multiple time step integration, and allows subsets of the Forces in a System to be evaluated at different times. By default, all Forces are in group 0. Call setForceGroup() to change this. Some Force subclasses may provide additional methods to further split their computations into multiple groups. Be aware that particular Platforms may place restrictions on the use of force groups, such as requiring all nonbonded forces to be in the same group.

Methods

 Force ~Force getForceGroup Get the force group this Force belongs to. setForceGroup Set the force group this Force belongs to. usesPeriodicBoundaryConditions Returns whether or not this force makes use of periodic boundary conditions.
Force()
~Force()
int getForceGroup() const

Get the force group this Force belongs to.

void setForceGroup(int group)

Set the force group this Force belongs to.

• group – the group index. Legal values are between 0 and 31 (inclusive).
bool usesPeriodicBoundaryConditions() const

Returns whether or not this force makes use of periodic boundary conditions. This method should be overridden for all Force subclasses, or a OpenMM::OpenMMException will be thrown

Returns: true if Force uses periodic boundaries or false if it does not