# %%
"""
.. _eos_example:

=======================
Equation of state (EOS)
=======================

First, do a bulk calculation for different lattice constants:
"""

# %%
import numpy as np

from ase import Atoms
from ase.calculators.emt import EMT
from ase.eos import EquationOfState
from ase.io import read
from ase.io.trajectory import Trajectory
from ase.units import kJ

a = 4.0  # approximate lattice constant
b = a / 2
ag = Atoms(
    'Ag', cell=[(0, b, b), (b, 0, b), (b, b, 0)], pbc=1, calculator=EMT()
)  # use EMT potential
cell = ag.get_cell()
traj = Trajectory('Ag.traj', 'w')
for x in np.linspace(0.95, 1.05, 5):
    ag.set_cell(cell * x, scale_atoms=True)
    ag.get_potential_energy()
    traj.write(ag)

# %%
# This writes a trajectory file containing five configurations of FCC silver
# for five different lattice constants. Now, analyse the result with
# the :class:`~ase.eos.EquationOfState` class:

configs = read('Ag.traj@0:5')  # read 5 configurations
# Extract volumes and energies:
volumes = [ag.get_volume() for ag in configs]
energies = [ag.get_potential_energy() for ag in configs]
eos = EquationOfState(volumes, energies)
v0, e0, B = eos.fit()
print(B / kJ * 1.0e24, 'GPa')
eos.plot('Ag-eos.png')

# %%
# A quicker way to do this analysis is to use the :mod:`ase.gui` tool::
#
#     $   ase gui Ag.traj

# %%
# And then choose :menuselection:`Tools --> Bulk modulus`.