# `VariableLangevinIntegrator`¶

class VariableLangevinIntegrator : public OpenMM::Integrator

This is an error controlled, variable time step Integrator that simulates a System using Langevin dynamics. It compares the result of the Langevin integrator to that of an explicit Euler integrator, takes the difference between the two as a measure of the integration error in each time step, and continuously adjusts the step size to keep the error below a specified tolerance. This both improves the stability of the integrator and allows it to take larger steps on average, while still maintaining comparable accuracy to a fixed step size integrator.

It is best not to think of the error tolerance as having any absolute meaning. It is just an adjustable parameter that affects the step size and integration accuracy. You should try different values to find the largest one that produces a trajectory sufficiently accurate for your purposes. 0.001 is often a good starting point.

You can optionally set a maximum step size it will ever use. This is useful to prevent it from taking excessively large steps in usual situations, such as when the system is right at a local energy minimum.

Public Functions

VariableLangevinIntegrator(double temperature, double frictionCoeff, double errorTol)

Create a VariableLangevinIntegrator.

Parameters
• temperature – the temperature of the heat bath (in Kelvin)

• frictionCoeff – the friction coefficient which couples the system to the heat bath (in inverse picoseconds)

• errorTol – the error tolerance

inline double getTemperature() const

Get the temperature of the heat bath (in Kelvin).

Returns

the temperature of the heat bath, measured in Kelvin

void setTemperature(double temp)

Set the temperature of the heat bath (in Kelvin).

Parameters

temp – the temperature of the heat bath, measured in Kelvin

inline double getFriction() const

Get the friction coefficient which determines how strongly the system is coupled to the heat bath (in inverse ps).

Returns

the friction coefficient, measured in 1/ps

void setFriction(double coeff)

Set the friction coefficient which determines how strongly the system is coupled to the heat bath (in inverse ps).

Parameters

coeff – the friction coefficient, measured in 1/ps

inline double getErrorTolerance() const

Get the error tolerance.

void setErrorTolerance(double tol)

Set the error tolerance.

inline double getMaximumStepSize() const

Get the maximum step size the integrator will ever use, in ps. If this is 0 (the default), no limit will be applied to step sizes.

void setMaximumStepSize(double size)

Set the maximum step size the integrator will ever use, in ps. If this is 0 (the default), no limit will be applied to step sizes.

inline int getRandomNumberSeed() const

Get the random number seed. See setRandomNumberSeed() for details.

inline void setRandomNumberSeed(int seed)

Set the random number seed. The precise meaning of this parameter is undefined, and is left up to each Platform to interpret in an appropriate way. It is guaranteed that if two simulations are run with different random number seeds, the sequence of random forces will be different. On the other hand, no guarantees are made about the behavior of simulations that use the same seed. In particular, Platforms are permitted to use non-deterministic algorithms which produce different results on successive runs, even if those runs were initialized identically.

If seed is set to 0 (which is the default value assigned), a unique seed is chosen when a Context is created from this Force. This is done to ensure that each Context receives unique random seeds without you needing to set them explicitly.

virtual void step(int steps)

Advance a simulation through time by taking a series of time steps.

Parameters

steps – the number of time steps to take

void stepTo(double time)

Advance a simulation through time by taking a series of steps until a specified time is reached. When this method returns, the simulation time will exactly equal the time which was specified. If you call this method and specify a time that is earlier than the current time, it will return without doing anything.

Parameters

time – the time to which the simulation should be advanced