# CustomBondForce¶

class OpenMM::CustomBondForce

This class implements bonded interactions between pairs of particles. Unlike HarmonicBondForce, the functional form of the interaction is completely customizable, and may involve arbitrary algebraic expressions. It may depend on the distance between particles, as well as on arbitrary global and per-bond parameters.

To use this class, create a CustomBondForce object, passing an algebraic expression to the constructor that defines the interaction energy between each pair of bonded particles. The expression may depend on r, the distance between the particles, as well as on any parameters you choose. Then call addPerBondParameter() to define per-bond parameters, and addGlobalParameter() to define global parameters. The values of per-bond 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 addBond() once for each bond. After a bond has been added, you can modify its parameters by calling setBondParameters(). This will have no effect on Contexts that already exist unless you call updateParametersInContext().

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

CustomBondForce* force = new CustomBondForce("0.5*k*(r-r0)^2");


This force depends on two parameters: the spring constant k and equilibrium distance r0. The following code defines these parameters:

force->addPerBondParameter("k");


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, 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.

Methods

 CustomBondForce Create a CustomBondForce. getNumBonds Get the number of bonds for which force field parameters have been defined. getNumPerBondParameters Get the number of per-bond parameters that the interaction depends on. getNumGlobalParameters Get the number of global parameters that the interaction depends on. getNumEnergyParameterDerivatives Get the number of global parameters with respect to which the derivative of the energy should be computed. getEnergyFunction Get the algebraic expression that gives the interaction energy for each bond setEnergyFunction Set the algebraic expression that gives the interaction energy for each bond addPerBondParameter Add a new per-bond parameter that the interaction may depend on. getPerBondParameterName Get the name of a per-bond parameter. setPerBondParameterName Set the name of a per-bond parameter. addGlobalParameter Add a new global parameter that the interaction may depend on. getGlobalParameterName Get the name of a global parameter. setGlobalParameterName Set the name of a global parameter. getGlobalParameterDefaultValue Get the default value of a global parameter. setGlobalParameterDefaultValue Set the default value of a global parameter. addEnergyParameterDerivative Request that this Force compute the derivative of its energy with respect to a global parameter. getEnergyParameterDerivativeName Get the name of a global parameter with respect to which this Force should compute the derivative of the energy. addBond Add a bond term to the force field. getBondParameters Get the force field parameters for a bond term. setBondParameters Set the force field parameters for a bond term. updateParametersInContext Update the per-bond 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.
CustomBondForce(const std::string &energy)

Create a CustomBondForce().

• energy – an algebraic expression giving the interaction energy between two bonded particles as a function of r, the distance between them
int getNumBonds() const

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

int getNumPerBondParameters() const

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

int getNumGlobalParameters() const

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

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 bond

void setEnergyFunction(const std::string &energy)

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

int addPerBondParameter(const std::string &name)

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

• name – the name of the parameter
Returns: the index of the parameter that was added
const std::string &getPerBondParameterName(int index) const

Get the name of a per-bond parameter.

• index – the index of the parameter for which to get the name
Returns: the parameter name
void setPerBondParameterName(int index, const std::string &name)

Set the name of a per-bond parameter.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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().

• 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.

Returns: the parameter name
int addBond(int particle1, int particle2, const std::vector<double> &parameters = std::vector< double >())

Add a bond term to the force field.

• particle1 – the index of the first particle connected by the bond
• particle2 – the index of the second particle connected by the bond
• parameters – the list of parameters for the new bond
Returns: the index of the bond that was added
void getBondParameters(int index, int &particle1, int &particle2, std::vector<double> &parameters) const

Get the force field parameters for a bond term.

• index – the index of the bond for which to get parameters
• particle1 – [out] the index of the first particle connected by the bond
• particle2 – [out] the index of the second particle connected by the bond
• parameters – [out] the list of parameters for the bond
void setBondParameters(int index, int particle1, int particle2, const std::vector<double> &parameters = std::vector< double >())

Set the force field parameters for a bond term.

• index – the index of the bond for which to set parameters
• particle1 – the index of the first particle connected by the bond
• particle2 – the index of the second particle connected by the bond
• parameters – the list of parameters for the bond
void updateParametersInContext(Context &context)

Update the per-bond 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 setBondParameters() 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-bond 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 bond cannot be changed, nor can new bonds 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