Choosing the integrator

Perhaps the next most important choice is what kind of integration algorithm to use. Above we did a constant-temperature (NVT) algorithm (the actual algorithm is of the Nosé-Hoover type). For constant energy (NVE) runs we create the integrator as follows, here passing only the time-step:

itg = IntegratorNVE(timeStep=0.0025)

(Technical note: this creates an object of the same type as IntegratorNVT, but here it defaults to NVE mode--in fact in either case one can switch thermostatting on or off using the SetThermostatOn() method).

Additional integrators available in RUMD are listed below

Name of Integrator Description Arguments
IntegratorNVE The NVE Leap-frog algorithm. timeStep
IntegratorNVT The Nos$\acute{e}$-Hoover NVT algorithm. timeStep, targetTemperature
IntegratorNPTAtomic NPT algorithm. (JCP 101, N 5, 1994)
timeStep, targetTemperature, thermostatRelaxationTime, targetPressure, barostatRelaxationTime
IntegratorNVU Algorithm conserving the total potential energy. dispLength, potentialEnergy
IntegratorMMC Metropolis NVT Monte Carlo. dispLength, targetTemperature
IntegratorIHS Energy minimization via the Leap-frog algorithm. timeStep
IntegratorSLLOD Shear an atomic system in the xy-plane using the SLLOD equations. timeStep, strainRate
IntegratorMolecularSLLOD Shear a molecular system in the xy-plane using the SLLOD equations. timeStep, strainRate

The above integrators are chosen in the usual way with named arguments as given in the table. In the case of IntegratorNPTAtomic the user must choose suitable relaxation times for the thermostat and the barostat. An example of reasonable values for the thermostat and barostat relaxation times for the LJ system at $T=2.0$ and $p=5.0$ are thermostatRelaxationTime=0.4 and barostatRelaxationTime=10.0.

Heine Larsen 2017-07-21