# PeriodicTorsionForce¶

class OpenMM::PeriodicTorsionForce

This class implements an interaction between groups of four particles that varies periodically with the torsion angle between them. To use it, create a PeriodicTorsionForce() object then call addTorsion() once for each torsion. After a torsion has been added, you can modify its force field parameters by calling setTorsionParameters(). This will have no effect on Contexts that already exist unless you call updateParametersInContext().

Methods

 PeriodicTorsionForce() Create a PeriodicTorsionForce(). getNumTorsions() Get the number of periodic torsion terms in the potential function addTorsion() Add a periodic torsion term to the force field. getTorsionParameters() Get the force field parameters for a periodic torsion term. setTorsionParameters() Set the force field parameters for a periodic torsion term. updateParametersInContext() Update the per-torsion parameters in a Context to match those stored in this Force object. usesPeriodicBoundaryConditions() Returns whether or not this force makes use of periodic boundary conditions.
PeriodicTorsionForce()

Create a PeriodicTorsionForce().

int getNumTorsions() const

Get the number of periodic torsion terms in the potential function

int addTorsion(int particle1, int particle2, int particle3, int particle4, int periodicity, double phase, double k)

Add a periodic torsion term to the force field.

• particle1 – the index of the first particle forming the torsion
• particle2 – the index of the second particle forming the torsion
• particle3 – the index of the third particle forming the torsion
• particle4 – the index of the fourth particle forming the torsion
• periodicity – the periodicity of the torsion
• phase – the phase offset of the torsion, measured in radians
• k – the force constant for the torsion
Returns: the index of the torsion that was added
void getTorsionParameters(int index, int &particle1, int &particle2, int &particle3, int &particle4, int &periodicity, double &phase, double &k) const

Get the force field parameters for a periodic torsion term.

• index – the index of the torsion for which to get parameters
• particle1 – [out] the index of the first particle forming the torsion
• particle2 – [out] the index of the second particle forming the torsion
• particle3 – [out] the index of the third particle forming the torsion
• particle4 – [out] the index of the fourth particle forming the torsion
• periodicity – [out] the periodicity of the torsion
• phase – [out] the phase offset of the torsion, measured in radians
• k – [out] the force constant for the torsion
void setTorsionParameters(int index, int particle1, int particle2, int particle3, int particle4, int periodicity, double phase, double k)

Set the force field parameters for a periodic torsion term.

• index – the index of the torsion for which to set parameters
• particle1 – the index of the first particle forming the torsion
• particle2 – the index of the second particle forming the torsion
• particle3 – the index of the third particle forming the torsion
• particle4 – the index of the fourth particle forming the torsion
• periodicity – the periodicity of the torsion
• phase – the phase offset of the torsion, measured in radians
• k – the force constant for the torsion
void updateParametersInContext(Context &context)

Update the per-torsion parameters in a Context to match those stored in this Force object. This method provides an efficient method to update certain parameters in an existing Context without needing to reinitialize it. Simply call setTorsionParameters() to modify this object’s parameters, then call updateParametersInContext() to copy them over to the Context.

The only information this method updates is the values of per-torsion parameters. The set of particles involved in a torsion cannot be changed, nor can new torsions be added.

bool usesPeriodicBoundaryConditions() const

Returns whether or not this force makes use of periodic boundary conditions.

Returns: true if force uses PBC and false otherwise