# Context¶

class OpenMM::Context

A Context stores the complete state of a simulation. More specifically, it includes:

You can retrieve a snapshot of the current state at any time by calling getState(). This allows you to record the state of the simulation at various points, either for analysis or for checkpointing. getState() can also be used to retrieve the current forces on each particle and the current energy of the System.

Methods

 Context Construct a new Context in which to run a simulation. Context Construct a new Context in which to run a simulation, explicitly specifying what Platform should be used to perform calculations. Context Construct a new Context in which to run a simulation, explicitly specifying what Platform should be used to perform calculations and the values of platform-specific properties. ~Context getSystem Get System being simulated in this context. getIntegrator Get Integrator being used to by this context. getIntegrator Get Integrator being used to by this context. getPlatform Get the Platform being used for calculations. getPlatform Get the Platform being used for calculations. getState Get a State object recording the current state information stored in this context. setState Copy information from a State object into this Context. setTime Set the current time of the simulation (in picoseconds). setPositions Set the positions of all particles in the System (measured in nm). setVelocities Set the velocities of all particles in the System (measured in nm/picosecond). setVelocitiesToTemperature Set the velocities of all particles in the System to random values chosen from a Boltzmann distribution at a given temperature. getParameters Get all adjustable parameters that have been defined by Force objects in the System, along with their current values. getParameter Get the value of an adjustable parameter defined by a Force object in the System. setParameter Set the value of an adjustable parameter defined by a Force object in the System. setPeriodicBoxVectors Set the vectors defining the axes of the periodic box (measured in nm). applyConstraints Update the positions of particles so that all distance constraints are satisfied. applyVelocityConstraints Update the velocities of particles so the net velocity of each constrained distance is zero. computeVirtualSites Recompute the locations of all virtual sites. reinitialize When a Context is created, it caches information about the System being simulated and the Force objects contained in it. createCheckpoint Create a checkpoint recording the current state of the Context. loadCheckpoint Load a checkpoint that was written by createCheckpoint(). getMolecules Get a description of how the particles in the system are grouped into molecules.
Context(const System &system, Integrator &integrator)

Construct a new Context() in which to run a simulation.

Context(const System &system, Integrator &integrator, Platform &platform)

Construct a new Context() in which to run a simulation, explicitly specifying what Platform should be used to perform calculations.

Context(const System &system, Integrator &integrator, Platform &platform, const std::map<std::string, std::string> &properties)

Construct a new Context() in which to run a simulation, explicitly specifying what Platform should be used to perform calculations and the values of platform-specific properties.

~Context()
const System &getSystem() const

Get System being simulated in this context.

const Integrator &getIntegrator() const

Get Integrator being used to by this context.

Integrator &getIntegrator()

Get Integrator being used to by this context.

const Platform &getPlatform() const

Get the Platform being used for calculations.

Platform &getPlatform()

Get the Platform being used for calculations.

State getState(int types, bool enforcePeriodicBox = false, int groups = 0xFFFFFFFF) const

Get a State object recording the current state information stored in this context.

• 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 in the Context, 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.
void setState(const State &state)

Copy information from a State object into this Context. This restores the Context to approximately the same state it was in when the State was created. If the State does not include a piece of information (e.g. positions or velocities), that aspect of the Context is left unchanged.

Even when all possible information is included in the State, the effect of calling this method is still less complete than loadCheckpoint(). For example, it does not restore the internal states of random number generators. On the other hand, it has the advantage of not being hardware specific.

void setTime(double time)

Set the current time of the simulation (in picoseconds).

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

Set the positions of all particles in the System (measured in nm). This method simply sets the positions without checking to see whether they satisfy distance constraints. If you want constraints to be enforced, call applyConstraints() after setting the positions.

• positions – a vector whose length equals the number of particles in the System. The i’th element contains the position of the i’th particle.
void setVelocities(const std::vector<Vec3> &velocities)

Set the velocities of all particles in the System (measured in nm/picosecond).

• velocities – a vector whose length equals the number of particles in the System. The i’th element contains the velocity of the i’th particle.
void setVelocitiesToTemperature(double temperature, int randomSeed = osrngseed())

Set the velocities of all particles in the System to random values chosen from a Boltzmann distribution at a given temperature.

• temperature – the temperature for which to select the velocities (measured in Kelvin)
• randomSeed – the random number seed to use when selecting velocities
const std::map<std::string, double> &getParameters() const

Get all adjustable parameters that have been defined by Force objects in the System, along with their current values.

double getParameter(const std::string &name) const

Get the value of an adjustable parameter defined by a Force object in the System.

• name – the name of the parameter to get
void setParameter(const std::string &name, double value)

Set the value of an adjustable parameter defined by a Force object in the System.

• name – the name of the parameter to set
• value – the value of the parameter
void setPeriodicBoxVectors(const Vec3 &a, const Vec3 &b, const Vec3 &c)

Set the vectors defining the axes of the periodic box (measured in nm). They will affect any Force that uses periodic boundary conditions.

Triclinic boxes are supported, but the vectors must satisfy certain requirements. In particular, a must point in the x direction, b must point “mostly” in the y direction, and c must point “mostly” in the z direction. See the documentation for details.

• a – the vector defining the first edge of the periodic box
• b – the vector defining the second edge of the periodic box
• c – the vector defining the third edge of the periodic box
void applyConstraints(double tol)

Update the positions of particles so that all distance constraints are satisfied. This also recomputes the locations of all virtual sites.

• tol – the distance tolerance within which constraints must be satisfied.
void applyVelocityConstraints(double tol)

Update the velocities of particles so the net velocity of each constrained distance is zero.

• tol – the velocity tolerance within which constraints must be satisfied.
void computeVirtualSites()

Recompute the locations of all virtual sites. There is rarely a reason to call this, since virtual sites are also updated by applyConstraints(). This is only for the rare situations when you want to enforce virtual sites but

void reinitialize(bool preserveState = false)

When a Context is created, it caches information about the System being simulated and the Force objects contained in it. This means that, if the System or Forces are then modified, the Context does not see the changes. Call reinitialize() to force the Context to rebuild its internal representation of the System and pick up any changes that have been made.

This is an expensive operation, so you should try to avoid calling it too frequently. Most Force classes have an updateParametersInContext() method that provides a less expensive way of updating certain types of information. However, this method is the only way to make some types of changes, so it is sometimes necessary to call it.

By default, reinitializing a Context causes all state information (positions, velocities, etc.) to be discarded. You can optionally tell it to try to preserve state information. It does this by internally creating a checkpoint, then reinitializing the Context, then loading the checkpoint. Be aware that if the System has changed in a way that prevents the checkpoint from being loaded (such as changing the number of particles), this will throw an exception and the state information will be lost.

void createCheckpoint(std::ostream &stream)

Create a checkpoint recording the current state of the Context. This should be treated as an opaque block of binary data. See loadCheckpoint() for more details.

• stream – an output stream the checkpoint data should be written to
void loadCheckpoint(std::istream &stream)

Load a checkpoint that was written by createCheckpoint().

A checkpoint contains not only publicly visible data such as the particle positions and velocities, but also internal data such as the states of random number generators. Ideally, loading a checkpoint should restore the Context to an identical state to when it was written, such that continuing the simulation will produce an identical trajectory. This is not strictly guaranteed to be true, however, and should not be relied on. For most purposes, however, the internal state should be close enough to be reasonably considered equivalent.

A checkpoint contains data that is highly specific to the Context from which it was created. It depends on the details of the System, the Platform being used, and the hardware and software of the computer it was created on. If you try to load it on a computer with different hardware, or for a System that is different in any way, loading is likely to fail. Checkpoints created with different versions of OpenMM are also often incompatible. If a checkpoint cannot be loaded, that is signaled by throwing an exception.

• stream – an input stream the checkpoint data should be read from
const std::vector<std::vector<int>> &getMolecules() const

Get a description of how the particles in the system are grouped into molecules. Two particles are in the same molecule if they are connected by constraints or bonds, where every Force object can define bonds in whatever way are appropriate to that force.

Each element lists the indices of all particles in a single molecule. Every particle is guaranteed to belong to exactly one molecule.