CustomAngleForce

class CustomAngleForce : public OpenMM::Force

This class implements interactions between sets of three particles that depend on the angle between them. Unlike HarmonicAngleForce, the functional form of the interaction is completely customizable, and may involve arbitrary algebraic expressions. In addition to the angle formed by the particles, it may depend on arbitrary global and per-angle parameters.

To use this class, create a CustomAngleForce object, passing an algebraic expression to the constructor that defines the interaction energy between each set of particles. The expression may depend on theta, the angle formed by the particles, as well as on any parameters you choose. Then call addPerAngleParameter() to define per-angle parameters, and addGlobalParameter() to define global parameters. The values of per-angle parameters are specified as part of the system definition, while values of global parameters may be modified during a simulation by calling Context::setParameter(). Finally, call addAngle() once for each angle. After an angle has been added, you can modify its parameters by calling setAngleParameters(). This will have no effect on Contexts that already exist unless you call updateParametersInContext().

As an example, the following code creates a CustomAngleForce that implements a harmonic potential:

CustomAngleForce* force = new CustomAngleForce("0.5*k*(theta-theta0)^2");

This force depends on two parameters: the spring constant k and equilibrium angle theta0. The following code defines these parameters:

force->addPerAngleParameter("k");
force->addPerAngleParameter("theta0");

This class also has the ability to compute derivatives of the potential energy with respect to global parameters. Call addEnergyParameterDerivative() to request that the derivative with respect to a particular parameter be computed. You can then query its value in a Context by calling getState() on it.

Expressions may involve the operators + (add), - (subtract), * (multiply), / (divide), and ^ (power), and the following functions: sqrt, exp, log, sin, cos, sec, csc, tan, cot, asin, acos, atan, atan2, sinh, cosh, tanh, erf, erfc, min, max, abs, floor, ceil, step, delta, select. All trigonometric functions are defined in radians, and log is the natural logarithm. step(x) = 0 if x is less than 0, 1 otherwise. delta(x) = 1 if x is 0, 0 otherwise. select(x,y,z) = z if x = 0, y otherwise.

Public Functions

explicit CustomAngleForce(const std::string &energy)

Create a CustomAngleForce.

Parameters

energy – an algebraic expression giving the interaction energy between three particles as a function of theta, the angle between them

inline int getNumAngles() const

Get the number of angles for which force field parameters have been defined.

inline int getNumPerAngleParameters() const

Get the number of per-angle parameters that the interaction depends on.

inline int getNumGlobalParameters() const

Get the number of global parameters that the interaction depends on.

inline int getNumEnergyParameterDerivatives() const

Get the number of global parameters with respect to which the derivative of the energy should be computed.

const std::string &getEnergyFunction() const

Get the algebraic expression that gives the interaction energy for each angle

void setEnergyFunction(const std::string &energy)

Set the algebraic expression that gives the interaction energy for each angle

int addPerAngleParameter(const std::string &name)

Add a new per-angle parameter that the interaction may depend on.

Parameters

name – the name of the parameter

Returns

the index of the parameter that was added

const std::string &getPerAngleParameterName(int index) const

Get the name of a per-angle parameter.

Parameters

index – the index of the parameter for which to get the name

Returns

the parameter name

void setPerAngleParameterName(int index, const std::string &name)

Set the name of a per-angle parameter.

Parameters

• index – the index of the parameter for which to set the name

• name – the name of the parameter

int addGlobalParameter(const std::string &name, double defaultValue)

Add a new global parameter that the interaction may depend on. The default value provided to this method is the initial value of the parameter in newly created Contexts. You can change the value at any time by calling setParameter() on the Context.

Parameters

• name – the name of the parameter

• defaultValue – the default value of the parameter

Returns

the index of the parameter that was added

const std::string &getGlobalParameterName(int index) const

Get the name of a global parameter.

Parameters

index – the index of the parameter for which to get the name

Returns

the parameter name

void setGlobalParameterName(int index, const std::string &name)

Set the name of a global parameter.

Parameters

• index – the index of the parameter for which to set the name

• name – the name of the parameter

double getGlobalParameterDefaultValue(int index) const

Get the default value of a global parameter.

Parameters

index – the index of the parameter for which to get the default value

Returns

the parameter default value

void setGlobalParameterDefaultValue(int index, double defaultValue)

Set the default value of a global parameter.

Parameters

• index – the index of the parameter for which to set the default value

• defaultValue – the default value of the parameter

void addEnergyParameterDerivative(const std::string &name)

Request that this Force compute the derivative of its energy with respect to a global parameter. The parameter must have already been added with addGlobalParameter().

Parameters

name – the name of the parameter

const std::string &getEnergyParameterDerivativeName(int index) const

Get the name of a global parameter with respect to which this Force should compute the derivative of the energy.

Parameters

index – the index of the parameter derivative, between 0 and getNumEnergyParameterDerivatives()

Returns

the parameter name

int addAngle(int particle1, int particle2, int particle3, const std::vector<double> &parameters = std::vector<double>())

Add an angle term to the force field.

Parameters

• particle1 – the index of the first particle connected by the angle

• particle2 – the index of the second particle connected by the angle

• particle3 – the index of the third particle connected by the angle

• parameters – the list of parameters for the new angle

Returns

the index of the angle that was added

void getAngleParameters(int index, int &particle1, int &particle2, int &particle3, std::vector<double> &parameters) const

Get the force field parameters for an angle term.

Parameters

• index – the index of the angle for which to get parameters

• particle1 – [out] the index of the first particle connected by the angle

• particle2 – [out] the index of the second particle connected by the angle

• particle3 – [out] the index of the third particle connected by the angle

• parameters – [out] the list of parameters for the angle

void setAngleParameters(int index, int particle1, int particle2, int particle3, const std::vector<double> &parameters = std::vector<double>())

Set the force field parameters for an angle term.

Parameters

• index – the index of the angle for which to set parameters

• particle1 – the index of the first particle connected by the angle

• particle2 – the index of the second particle connected by the angle

• particle3 – the index of the third particle connected by the angle

• parameters – the list of parameters for the angle

void updateParametersInContext(Context &context)

Update the per-angle 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 setAngleParameters() to modify this object’s parameters, then call updateParametersInContext() to copy them over to the Context.

This method has several limitations. The only information it updates is the values of per-angle parameters. All other aspects of the Force (such as the energy function) are unaffected and can only be changed by reinitializing the Context. The set of particles involved in a angle cannot be changed, nor can new angles 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.

virtual bool usesPeriodicBoundaryConditions() const

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

Returns

true if force uses PBC and false otherwise