# CompoundIntegrator¶

class OpenMM::CompoundIntegrator

This class allows you to use multiple integration algorithms within a single simulation, switching back and forth between them. To use it, create whatever other Integrators you need, then add all of them to a CustomIntegrator:

CompoundIntegrator compoundIntegrator;


Next create a Context, specifying the CompoundIntegrator as the Integrator to use for the Context:

Context context(system, compoundIntegrator);


Finally, call setCurrentIntegrator() to set which Integrator is active. That one will be used for all calls to step() until the next time you change it.

compoundIntegrator.setCurrentIntegrator(0);
compoundIntegrator.step(1000); // Take 1000 steps of Verlet dynamics
compoundIntegrator.setCurrentIntegrator(1);
compoundIntegrator.step(1000); // Take 1000 steps of Langevin dynamics


When switching between integrators, it is important to make sure they are compatible with each other, and that they will interpret the positions and velocities in the same way. Remember that leapfrog style integrators assume the positions and velocities are offset from each other by half a time step. When switching between a leapfrog and non-leapfrog integrator, you must first adjust the velocities to avoid introducing error. This is also true when switching between two leapfrog integrators that use different step sizes, since they will interpret the velocities as corresponding to different times.

Methods

 CompoundIntegrator Create a CompoundIntegrator. ~CompoundIntegrator getNumIntegrators Get the number of Integrators that have been added to this CompoundIntegrator. addIntegrator Add an Integrator to this CompoundIntegrator. getIntegrator Get a reference to one of the Integrators that have been added to this CompoundIntegrator. getIntegrator Get a const reference to one of the Integrators that have been added to this CompoundIntegrator. getCurrentIntegrator Get the index of the current Integrator. setCurrentIntegrator Set the current Integrator. getStepSize Get the size of each time step, in picoseconds. setStepSize Set the size of each time step, in picoseconds. getConstraintTolerance Get the distance tolerance within which constraints are maintained, as a fraction of the constrained distance. setConstraintTolerance Set the distance tolerance within which constraints are maintained, as a fraction of the constrained distance. step Advance a simulation through time by taking a series of time steps.
CompoundIntegrator()

Create a CompoundIntegrator().

~CompoundIntegrator()
int getNumIntegrators() const

Get the number of Integrators that have been added to this CompoundIntegrator.

int addIntegrator(Integrator *integrator)

Add an Integrator to this CompoundIntegrator. The Integrator object should have been created on the heap with the “new” operator. The CompoundIntegrator takes over ownership of it, and deletes it when the CompoundIntegrator itself is deleted. All Integrators must be added before the Context is created.

Returns: the index of the Integrator that was added
Integrator &getIntegrator(int index)

Get a reference to one of the Integrators that have been added to this CompoundIntegrator.

const Integrator &getIntegrator(int index) const

Get a const reference to one of the Integrators that have been added to this CompoundIntegrator.

int getCurrentIntegrator() const

Get the index of the current Integrator.

void setCurrentIntegrator(int index)

Set the current Integrator.

double getStepSize() const

Get the size of each time step, in picoseconds. This method calls getStepSize() on whichever Integrator has been set as current.

Returns: the step size, measured in ps
void setStepSize(double size)

Set the size of each time step, in picoseconds. This method calls setStepSize() on whichever Integrator has been set as current.

• size – the step size, measured in ps
double getConstraintTolerance() const

Get the distance tolerance within which constraints are maintained, as a fraction of the constrained distance. This method calls getConstraintTolerance() on whichever Integrator has been set as current.

void setConstraintTolerance(double tol)

Set the distance tolerance within which constraints are maintained, as a fraction of the constrained distance. This method calls setConstraintTolerance() on whichever Integrator has been set as current.

void step(int steps)

Advance a simulation through time by taking a series of time steps. This method calls step() on whichever Integrator has been set as current.

• steps – the number of time steps to take