openmmtools.testsystems.LennardJonesFluid

class openmmtools.testsystems.LennardJonesFluid(nparticles=1000, reduced_density=0.05, mass=Quantity(value=39.9, unit=dalton), sigma=Quantity(value=3.4, unit=angstrom), epsilon=Quantity(value=0.238, unit=kilocalorie/mole), cutoff=None, switch_width=Quantity(value=3.4, unit=angstrom), shift=False, dispersion_correction=True, lattice=False, charge=None, ewaldErrorTolerance=None, **kwargs)[source]

Create a periodic fluid of Lennard-Jones particles. Initial positions are assigned using a subrandom grid to minimize steric interactions.

Parameters:
nparticles : int, optional, default=1000

Number of Lennard-Jones particles.

reduced_density : float, optional, default=0.05

Reduced density (density * sigma**3); default is appropriate for gas

mass : simtk.unit.Quantity, optional, default=39.9 * unit.amu

mass of each particle; default is appropriate for argon

sigma : simtk.unit.Quantity, optional, default=3.4 * unit.angstrom

Lennard-Jones sigma parameter; default is appropriate for argon

epsilon : simtk.unit.Quantity, optional, default=0.238 * unit.kilocalories_per_mole

Lennard-Jones well depth; default is appropriate for argon

cutoff : simtk.unit.Quantity, optional, default=None

Cutoff for nonbonded interactions. If None, defaults to 3.0 * sigma

switch_width : simtk.unit.Quantity with units compatible with angstroms, optional, default=3.4 * unit.angstrom

switching function is turned on at cutoff - switch_width If None, no switch will be applied (e.g. hard cutoff). Ignored if shift=True.

shift : bool, optional, default=False

If True, will shift Lennard-Jones potential so energy will be continuous at cutoff (switch_width is ignored).

dispersion_correction : bool, optional, default=True

if True, will use analytical dispersion correction (if not using switching function)

lattice : bool, optional, default=False

If True, use fcc sphere packing to generate initial positions. The box size will be determined by nparticles and reduced_density.

charge : simtk.unit, optional, default=None

If not None, use alternating plus and minus charge for the particle charges. Also, if not None, use PME for electrostatics. Obviously this is no longer a traditional LJ system, but this option could be useful for testing the effect of charges in small systems.

ewaldErrorTolerance : float, optional, default=DEFAULT_EWALD_ERROR_TOLERANCE

The Ewald or PME tolerance. Used only if charge is not None.

Examples

Create default-size Lennard-Jones fluid.

>>> fluid = LennardJonesFluid()
>>> system, positions = fluid.system, fluid.positions

Create a larger box of Lennard-Jones particles with specified reduced density.

>>> fluid = LennardJonesFluid(nparticles=1000, reduced_density=0.50)
>>> system, positions = fluid.system, fluid.positions

Create Lennard-Jones fluid using switched particle interactions (switched off betwee 7 and 9 A) and more particles.

>>> fluid = LennardJonesFluid(switch_width=2.0*unit.angstroms, cutoff=9.0*unit.angstroms)
>>> system, positions = fluid.system, fluid.positions

Create Lennard-Jones fluid using shifted potential.

>>> fluid = LennardJonesFluid(cutoff=9.0*unit.angstroms, shift=True)
>>> system, positions = fluid.system, fluid.positions
Attributes:
analytical_properties

A list of available analytical properties, accessible via ‘get_propertyname(thermodynamic_state)’ calls.

mdtraj_topology

The mdtraj.Topology object corresponding to the test system (read-only).

name

The name of the test system.

positions

The simtk.unit.Quantity object containing the particle positions, with units compatible with simtk.unit.nanometers.

system

The simtk.openmm.System object corresponding to the test system.

topology

The simtk.openmm.app.Topology object corresponding to the test system.

Methods

reduced_potential_expectation(…) Calculate the expected potential energy in state_sampled_from, divided by kB * T in state_evaluated_in.
serialize() Return the System and positions in serialized XML form.
__init__(nparticles=1000, reduced_density=0.05, mass=Quantity(value=39.9, unit=dalton), sigma=Quantity(value=3.4, unit=angstrom), epsilon=Quantity(value=0.238, unit=kilocalorie/mole), cutoff=None, switch_width=Quantity(value=3.4, unit=angstrom), shift=False, dispersion_correction=True, lattice=False, charge=None, ewaldErrorTolerance=None, **kwargs)[source]

Abstract base class for test system.

Methods

__init__([nparticles, reduced_density, …]) Abstract base class for test system.
reduced_potential_expectation(…) Calculate the expected potential energy in state_sampled_from, divided by kB * T in state_evaluated_in.
serialize() Return the System and positions in serialized XML form.

Attributes

analytical_properties A list of available analytical properties, accessible via ‘get_propertyname(thermodynamic_state)’ calls.
mdtraj_topology The mdtraj.Topology object corresponding to the test system (read-only).
name The name of the test system.
positions The simtk.unit.Quantity object containing the particle positions, with units compatible with simtk.unit.nanometers.
system The simtk.openmm.System object corresponding to the test system.
topology The simtk.openmm.app.Topology object corresponding to the test system.
analytical_properties

A list of available analytical properties, accessible via ‘get_propertyname(thermodynamic_state)’ calls.

mdtraj_topology

The mdtraj.Topology object corresponding to the test system (read-only).

name

The name of the test system.

positions

The simtk.unit.Quantity object containing the particle positions, with units compatible with simtk.unit.nanometers.

reduced_potential_expectation(state_sampled_from, state_evaluated_in)

Calculate the expected potential energy in state_sampled_from, divided by kB * T in state_evaluated_in.

Notes

This is not called get_reduced_potential_expectation because this function requires two, not one, inputs.

serialize()

Return the System and positions in serialized XML form.

Returns:
system_xml : str

Serialized XML form of System object.

state_xml : str

Serialized XML form of State object containing particle positions.
system

The simtk.openmm.System object corresponding to the test system.

topology

The simtk.openmm.app.Topology object corresponding to the test system.