# OpenMM C++ API¶

The C++ API provides information about the classes and methods available in OpenMM for C++ developers. The public API is based on a small number of classes.

System: A System specifies generic properties of the system to be simulated: the number of particles it contains, the mass of each one, the size of the periodic box, etc. The interactions between the particles are specified through a set of Force objects (see below) that are added to the System. Force field specific parameters, such as particle charges, are not direct properties of the System. They are properties of the Force objects contained within the System.

Force: The Force objects added to a System define the behavior of the particles. Force is an abstract class; subclasses implement specific behaviors. The Force class is actually slightly more general than its name suggests. A Force can, indeed, apply forces to particles, but it can also directly modify particle positions and velocities in arbitrary ways. Some thermostats and barostats, for example, can be implemented as Force classes. Examples of Force subclasses include HarmonincBondForce, NonbondedForce, and MonteCarloBarostat.

Context: This stores all of the state information for a simulation: particle positions and velocities, as well as arbitrary parameters defined by the Forces in the System. It is possible to create multiple Contexts for a single System, and thus have multiple simulations of that System in progress at the same time.

Integrator: This implements an algorithm for advancing the simulation through time. It is an abstract class; subclasses implement specific algorithms. Examples of Integrator subclasses include LangevinIntegrator, VerletIntegrator, and BrownianIntegrator.

State: A State stores a snapshot of the simulation at a particular point in time. It is created by calling a method on a Context. This is the only way to query the values of state variables, such as particle positions and velocities; Context does not provide methods for accessing them directly.