ForceBiasMonteCarlo

class ForceBiasMonteCarlo(temperature=None, max_atom_displacement=None, heating_rate=None, random_seed=None, max_random_attempts=None)

Set up the ForceBiasMonteCarlo object.

Parameters:

  • temperature (PhysicalQuantity of type temperature | None) – The temperature at which the Monte Carlo simulation should be run.
    Default: 300*Kelvin

  • max_atom_displacement (PhysicalQuantity of type length | None) – The maximum distance an atom can move in each Cartesian direction during a single step.
    Default: 0.1*Angstrom

  • heating_rate (PhysicalQuantity of type temperature | None) – The change in temperature per step.
    Default: 0*Kelvin

  • random_seed (int | None) – The seed for the random generator. Must be between 0 and 2**32.
    Default: The default random seed

  • max_random_attempts (int) – The maximum attempts used in the rejection sampling of the probability distribution.
    Default: 500

monteCarloStep(configuration, forces, stress, constraints=None)

Perform a Monte Carlo step and apply the new positions to the configuration.

Parameters:
monteCarloTimeStep()
Returns:

The average time that elapses between each step according to the time-stamped force bias MC (tfMC) formalism or None if not initialized.

Return type:

PhysicalQuantity of type time | None

randomSeed()
Returns:

The random seed used to initialize the random number generator.

Return type:

int

reservoirTemperature()
Returns:

The current reservoir temperature.

Return type:

PhysicalQuantity of type temperature

uniqueString()

Return a unique string representing the state of the object.

Usage Example

Use the ForceBiasMonteCarlo object to run a TimeStampedForceBiasMonteCarlo simulation:

# -------------------------------------------------------------
# Time-Stamped Force-Bias Monte Carlo
# -------------------------------------------------------------

method = ForceBiasMonteCarlo(
    reservoir_temperature=500.0*Kelvin,
    max_atom_displacement=0.3*Ang,
)

mc_trajectory = TimeStampedForceBiasMonteCarlo(
    bulk_configuration,
    constraints=[],
    trajectory_filename='tfmc_trajectory.hdf5',
    steps=500,
    log_interval=50,
    method=method
)

tfmc_example.py

Notes

The ForceBiasMonteCarlo class is used in the TimeStampedForceBiasMonteCarlo function to run a time-stamped force-bias Monte Carlo (TFMC) simulation [1].

A Monte Carlo step is taken by displacing each Cartesian component \(\alpha\) of the position of atom i by a distance \(\zeta_{i,\alpha}\Delta_i\). The maximum displacement magnitude for each atom is given by \(\Delta_i = \Delta \left(m_{\textrm{min}}/m_i\right)^{1/4}\) [2], where \(\Delta\) is specified by the argument max_atom_displacement, and \(m_i\) are the atomic masses.

\(\zeta_{i, \alpha}\) is sampled from the interval \([-1, 1]\) with a probability:

\[\begin{split}P(\zeta_{i,\alpha}) = \begin{cases} \frac{\exp(\gamma_{i,\alpha}(2\zeta_{i,\alpha} + 1)) - \exp(-\gamma_{i,\alpha})} {\exp(\gamma_{i,\alpha}) - \exp(-\gamma_{i,\alpha})} & \zeta_{i,\alpha} \in \left[-1, 0\right[ \\ \frac{\exp(\gamma_{i,\alpha}) - \exp(\gamma_{i,\alpha}(2\zeta_{i,\alpha} - 1))} {\exp(\gamma_{i,\alpha}) - \exp(-\gamma_{i,\alpha})} & \zeta_{i,\alpha} \in \left]0, 1\right] \\ \end{cases}\end{split}\]

where \(\gamma_{i,\alpha} = 0.5 F_{i, \alpha} \Delta/k_B T\), \(F_{i,\alpha}\) is the force component on atom i, and T is the temperature at which the configurations are sampled, specified by the argument reservoir_temperature.

By adjusting the max_atom_displacement parameter one can tune accuracy vs. efficiency of the simulation. A small value results in a more accurate sampling of the canonical ensemble, whereas a large value increases the efficiency at which the phase space is sampled. Typically, values in the range \(0.1 R_{eq} \, - \, 0.3 R_{eq}\) are a reasonable choice, where \(R_{eq}\) represents an equilibrium bond length in the system.

If a non-zero heating_rate is specified, the reservoir temperature will be changed by the given value after each Monte Carlo step, resulting in an increase or decrease of the temperature during sampling.