CustomBondForce

class CustomBondForce : public OpenMM::Force

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");
force->addPerBondParameter("r0");

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 CustomBondForce(const std::string &energy)

Create a CustomBondForce.

Parameters

energy – an algebraic expression giving the interaction energy between two bonded particles as a function of r, the distance between them

inline int getNumBonds() const

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

inline int getNumPerBondParameters() const

Get the number of per-bond 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 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.

Parameters

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.

Parameters

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.

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

Add a bond term to the force field.

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 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.

Parameters

  • 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.

Parameters

  • 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.

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