# RPMDIntegrator¶

class OpenMM::RPMDIntegrator

This is an Integrator which simulates a System using ring polymer molecular dynamics (RPMD). It simulates many copies of the System, with successive copies connected by harmonic springs to form a ring. This allows certain quantum mechanical effects to be efficiently simulated.

By default this Integrator applies a PILE thermostat to the system to simulate constant temperature dynamics. You can disable the thermostat by calling setApplyThermostat(false).

Because this Integrator simulates many copies of the System at once, it must be used differently from other Integrators. Instead of setting positions and velocities by calling methods of the Context, you should use the corresponding methods of the Integrator to set them for specific copies of the System. Similarly, you should retrieve state information for particular copies by calling getState() on the Integrator. Do not query the Context for state information.

You can optionally specify a set of “ring polymer contractions”, by which different force groups are evaluated on different numbers of copies, instead of computing every force on every copy. This can be much more efficient, since different forces may vary widely in how many times they must be evaluated to produce sufficient accuracy. For example, you might simulate a 32 copy ring polymer and evaluate bonded forces on every copy, but contract it down to only 6 copies for computing nonbonded interactions, and down to only a single copy (the centroid) for computing the reciprocal space part of PME.

Methods

 RPMDIntegrator Create a RPMDIntegrator. RPMDIntegrator Create a RPMDIntegrator. getNumCopies Get the number of copies of the system being simulated. getTemperature Get the temperature of the heat bath (in Kelvin). setTemperature Set the temperature of the heat bath (in Kelvin). getFriction Get the friction coefficient which determines how strongly the system is coupled to the heat bath (in inverse ps). setFriction Set the friction coefficient which determines how strongly the system is coupled to the heat bath (in inverse ps). getApplyThermostat Get whether a thermostat is applied to the system. setApplyThermostat Set whether a thermostat is applied to the system. getRandomNumberSeed Get the random number seed. setRandomNumberSeed Set the random number seed. getContractions Get the ring polymer contractions to use for evaluating different force groups. setPositions Set the positions of all particles in one copy of the system. setVelocities Get the velocities of all particles in one copy of the system. getState Get a State object recording the current state information about one copy of the system. getTotalEnergy Get the total energy of the ring polymer. step Advance a simulation through time by taking a series of time steps.
RPMDIntegrator(int numCopies, double temperature, double frictionCoeff, double stepSize)

Create a RPMDIntegrator().

• numCopies – the number of copies of the system that should be simulated
• temperature – the temperature of the heat bath (in Kelvin)
• frictionCoeff – the friction coefficient which couples the system to the heat bath (in inverse picoseconds)
• stepSize – the step size with which to integrator the system (in picoseconds)
RPMDIntegrator(int numCopies, double temperature, double frictionCoeff, double stepSize, const std::map<int, int> &contractions)

Create a RPMDIntegrator().

• numCopies – the number of copies of the system that should be simulated
• temperature – the temperature of the heat bath (in Kelvin)
• frictionCoeff – the friction coefficient which couples the system to the heat bath (in inverse picoseconds)
• stepSize – the step size with which to integrator the system (in picoseconds)
• contractions – the ring polymer contractions to use for evaluating different force groups. Each key in the map is the index of a force group, and the corresponding value is the number of copies to evaluate that force group on. If no entry is provided for a force group (the default), it is evaluated independently on every copy.
int getNumCopies() const

Get the number of copies of the system being simulated.

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).

• temp – the temperature of the heat bath, measured in Kelvin
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).

• coeff – the friction coefficient, measured in 1/ps
bool getApplyThermostat() const

Get whether a thermostat is applied to the system.

void setApplyThermostat(bool apply)

Set whether a thermostat is applied to the system.

int getRandomNumberSeed() const

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

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.

const std::map<int, int> &getContractions() const

Get the ring polymer contractions to use for evaluating different force groups. Each key in the map is the index of a force group, and the corresponding value is the number of copies to evaluate that force group on. If no entry is provided for a force group, it is evaluated independently on every copy.

void setPositions(int copy, const std::vector<Vec3> &positions)

Set the positions of all particles in one copy of the system.

• copy – the index of the copy for which to set positions
• positions – the positions of all particles in the system
void setVelocities(int copy, const std::vector<Vec3> &velocities)

Get the velocities of all particles in one copy of the system.

• copy – the index of the copy for which to set velocities
• velocities – the velocities of all particles in the system
State getState(int copy, int types, bool enforcePeriodicBox = false, int groups = 0xFFFFFFFF)

Get a State object recording the current state information about one copy of the system.

• copy – the index of the copy for which to retrieve state information
• types – the set of data types which should be stored in the State object. This should be a union of DataType values, e.g. (State::Positions | State::Velocities).
• enforcePeriodicBox – if false, the position of each particle will be whatever position is stored by the integrator, regardless of periodic boundary conditions. If true, particle positions will be translated so the center of every molecule lies in the same periodic box.
• groups – a set of bit flags for which force groups to include when computing forces and energies. Group i will be included if (groups&(1<<i)) != 0. The default value includes all groups.
double getTotalEnergy()

Get the total energy of the ring polymer. This includes the potential and kinetic energies of all copies, plus the potential energy of the harmonic springs that link copies together.

void step(int steps)

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

• steps – the number of time steps to take