# AmoebaPiTorsionForce¶

class OpenMM::AmoebaPiTorsionForce

This class implements the Amoeba pi-torsion interaction.

To use it, create an AmoebaPiTorsionForce object then call addPiTorsion() once for each torsion. After a torsion has been added, you can modify its force field parameters by calling setPiTorsionParameters(). This will have no effect on Contexts that already exist unless you call updateParametersInContext().

Methods

 AmoebaPiTorsionForce Create an AmoebaPiTorsionForce. getNumPiTorsions Get the number of pi torsion terms in the potential function addPiTorsion Add a torsion term to the force field. getPiTorsionParameters Get the force field parameters for a torsion term. setPiTorsionParameters Set the force field parameters for a pi torsion term. updateParametersInContext Update the per-torsion parameters in a Context to match those stored in this Force object. setUsesPeriodicBoundaryConditions Set whether this force should apply periodic boundary conditions when calculating displacements. usesPeriodicBoundaryConditions Returns whether or not this force makes use of periodic boundary conditions.
AmoebaPiTorsionForce()

Create an AmoebaPiTorsionForce().

int getNumPiTorsions() const

Get the number of pi torsion terms in the potential function

int addPiTorsion(int particle1, int particle2, int particle3, int particle4, int particle5, int particle6, double k)

Add a torsion term to the force field.

• particle1 – the index of the first particle connected by the torsion
• particle2 – the index of the second particle connected by the torsion
• particle3 – the index of the third particle connected by the torsion
• particle4 – the index of the fourth particle connected by the torsion
• particle5 – the index of the fifth particle connected by the torsion
• particle6 – the index of the sixth particle connected by the torsion
• k – the force constant for the torsion
Returns: the index of the torsion that was added
void getPiTorsionParameters(int index, int &particle1, int &particle2, int &particle3, int &particle4, int &particle5, int &particle6, double &k) const

Get the force field parameters for a torsion term.

• index – the index of the torsion for which to get parameters
• particle1 – [out] the index of the first particle connected by the torsion
• particle2 – [out] the index of the second particle connected by the torsion
• particle3 – [out] the index of the third particle connected by the torsion
• particle4 – [out] the index of the fourth particle connected by the torsion
• particle5 – [out] the index of the fifth particle connected by the torsion
• particle6 – [out] the index of the sixth particle connected by the torsion
• k – [out] the force constant for the torsion
void setPiTorsionParameters(int index, int particle1, int particle2, int particle3, int particle4, int particle5, int particle6, double k)

Set the force field parameters for a pi torsion term.

• index – the index of the torsion for which to set parameters
• particle1 – the index of the first particle connected by the torsion
• particle2 – the index of the second particle connected by the torsion
• particle3 – the index of the third particle connected by the torsion
• particle4 – the index of the fourth particle connected by the torsion
• particle5 – the index of the fifth particle connected by the torsion
• particle6 – the index of the sixth particle connected by the torsion
• 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 setPiTorsionParameters() 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.

void setUsesPeriodicBoundaryConditions(bool periodic)

Set whether this force should apply periodic boundary conditions when calculating displacements. Usually this is not appropriate for bonded forces, but there are situations when it can be useful.

bool usesPeriodicBoundaryConditions() const

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

Returns: true if force uses PBC and false otherwise