"""Dimer: Diffusion along rows"""
from math import sqrt

import numpy as np

from ase import Atom, Atoms
from ase.calculators.emt import EMT
from ase.constraints import FixAtoms
from ase.io import Trajectory
from ase.mep import DimerControl, MinModeAtoms, MinModeTranslate
from ase.optimize import QuasiNewton

# Setting up the initial image:
a = 4.0614
b = a / sqrt(2)
h = b / 2
initial = Atoms('Al2',
                positions=[(0, 0, 0),
                           (a / 2, b / 2, -h)],
                cell=(a, b, 2 * h),
                pbc=(1, 1, 0))
initial *= (2, 2, 2)
initial.append(Atom('Al', (a / 2, b / 2, 3 * h)))
initial.center(vacuum=4.0, axis=2)

N = len(initial)  # number of atoms

# Make a mask of zeros and ones that select fixed atoms - the two
# bottom layers:
mask = initial.positions[:, 2] - min(initial.positions[:, 2]) < 1.5 * h
constraint = FixAtoms(mask=mask)
initial.set_constraint(constraint)

# Calculate using EMT:
initial.calc = EMT()

# Relax the initial state:
QuasiNewton(initial).run(fmax=0.05)
e0 = initial.get_potential_energy()

traj = Trajectory('dimer_along.traj', 'w', initial)
traj.write()

# Making dimer mask list:
d_mask = [False] * (N - 1) + [True]

# Set up the dimer:
d_control = DimerControl(initial_eigenmode_method='displacement',
                         displacement_method='vector',
                         logfile=None,
                         mask=d_mask)
d_atoms = MinModeAtoms(initial, d_control)

# Displacement settings:
displacement_vector = np.zeros((N, 3))
# Strength of displacement along y axis = along row:
displacement_vector[-1, 1] = 0.001
# The direction of the displacement is set by the a in
# displacement_vector[-1, a], where a can be 0 for x, 1 for y and 2 for z.
d_atoms.displace(displacement_vector=displacement_vector)

# Converge to a saddle point:
dim_rlx = MinModeTranslate(d_atoms,
                           trajectory=traj,
                           logfile=None)
dim_rlx.run(fmax=0.001)

diff = initial.get_potential_energy() - e0
print('The energy barrier is %f eV.' % diff)