"""Demonstrates molecular dynamics with constant energy."""
from __future__ import print_function

from ase.lattice.cubic import FaceCenteredCubic
from ase.md.langevin import Langevin
from ase.io.trajectory import Trajectory
from ase import units

from asap3 import EMT  # Way too slow with ase.EMT !
size = 10

T = 1500  # Kelvin

# Set up a crystal
atoms = FaceCenteredCubic(directions=[[1, 0, 0], [0, 1, 0], [0, 0, 1]],
                          symbol="Cu",
                          size=(size, size, size),
                          pbc=False)

# Describe the interatomic interactions with the Effective Medium Theory
atoms.set_calculator(EMT())

# We want to run MD with constant energy using the Langevin algorithm
# with a time step of 5 fs, the temperature T and the friction
# coefficient to 0.02 atomic units.
dyn = Langevin(atoms, 5 * units.fs, T * units.kB, 0.002)


def printenergy(a=atoms):  # store a reference to atoms in the definition.
    """Function to print the potential, kinetic and total energy."""
    epot = a.get_potential_energy() / len(a)
    ekin = a.get_kinetic_energy() / len(a)
    print('Energy per atom: Epot = %.3feV  Ekin = %.3feV (T=%3.0fK)  '
          'Etot = %.3feV' % (epot, ekin, ekin / (1.5 * units.kB), epot + ekin))
    
dyn.attach(printenergy, interval=50)

# We also want to save the positions of all atoms after every 100th time step.
traj = Trajectory('moldyn3.traj', 'w', atoms)
dyn.attach(traj.write, interval=50)

# Now run the dynamics
printenergy()
dyn.run(5000)
