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import numpy as np
from ase.md.md import MolecularDynamics
import warnings
class VelocityVerlet(MolecularDynamics):
def __init__(self, atoms, timestep=None, trajectory=None, logfile=None,
loginterval=1, dt=None, append_trajectory=False):
"""Molecular Dynamics object.
Parameters:
atoms: Atoms object
The Atoms object to operate on.
timestep: float
The time step in ASE time units.
trajectory: Trajectory object or str (optional)
Attach trajectory object. If *trajectory* is a string a
Trajectory will be constructed. Default: None.
logfile: file object or str (optional)
If *logfile* is a string, a file with that name will be opened.
Use '-' for stdout. Default: None.
loginterval: int (optional)
Only write a log line for every *loginterval* time steps.
Default: 1
append_trajectory: boolean
Defaults to False, which causes the trajectory file to be
overwriten each time the dynamics is restarted from scratch.
If True, the new structures are appended to the trajectory
file instead.
dt: float (deprecated)
Alias for timestep.
"""
if dt is not None:
warnings.warn(FutureWarning('dt variable is deprecated; please use timestep.'))
timestep = dt
if timestep is None:
raise TypeError('Missing timestep argument')
MolecularDynamics.__init__(self, atoms, timestep, trajectory, logfile,
loginterval,
append_trajectory=append_trajectory)
def step(self, forces=None):
atoms = self.atoms
if forces is None:
forces = atoms.get_forces()
p = atoms.get_momenta()
p += 0.5 * self.dt * forces
masses = atoms.get_masses()[:, np.newaxis]
r = atoms.get_positions()
# if we have constraints then this will do the first part of the
# RATTLE algorithm:
atoms.set_positions(r + self.dt * p / masses)
if atoms.constraints:
p = (atoms.get_positions() - r) * masses / self.dt
# We need to store the momenta on the atoms before calculating
# the forces, as in a parallel Asap calculation atoms may
# migrate during force calculations, and the momenta need to
# migrate along with the atoms.
atoms.set_momenta(p, apply_constraint=False)
forces = atoms.get_forces(md=True)
# Second part of RATTLE will be done here:
atoms.set_momenta(atoms.get_momenta() + 0.5 * self.dt * forces)
return forces
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