# Set up lattice
vector_a = [0.0, 2.7153, 2.7153]*Angstrom
vector_b = [2.7153, 0.0, 2.7153]*Angstrom
vector_c = [2.7153, 2.7153, 0.0]*Angstrom
lattice = UnitCell(vector_a, vector_b, vector_c)

# Define elements
elements = [Silicon, Silicon]

# Define coordinates
fractional_coordinates = [[ 0.  ,  0.  ,  0.  ],
                          [ 0.25,  0.25,  0.25]]

# Set up configuration
bulk_configuration = BulkConfiguration(
    bravais_lattice=lattice,
    elements=elements,
    fractional_coordinates=fractional_coordinates
    )

# -------------------------------------------------------------
# Calculator
# -------------------------------------------------------------

potentialSet = StillingerWeber_Si_1985()
calculator = TremoloXCalculator(parameters=potentialSet)

bulk_configuration.setCalculator(calculator)

# -------------------------------------------------------------
# Optimize Geometry
# -------------------------------------------------------------

bulk_configuration = OptimizeGeometry(
        bulk_configuration,
        max_forces=0.001*eV/Ang,
        max_stress=0.0001*eV/Ang**3,
        max_steps=200,
        max_step_length=0.2*Ang,
        constraints=[],
        trajectory_filename='opt2.nc',
        optimizer_method=LBFGS(),
        target_stress=[2.0, 0.0, 0.0, 0.0, 0.0, 0.0]*GPa,
        )

# Save the relaxed configuration
nlsave('optimized_configuration2.nc', bulk_configuration)