ElasticConstants¶

class ElasticConstants(configuration, optimizer=<NL.Dynamics.Optimization.OptimizerMethod.LBFGS object>, max_forces=None, max_steps=None, max_step_length=None, eta_max=None, n_eta=None, enable_symmetry=None, fit_order=None)¶

Class to calculate the elastic constants of a bulk material. Uses finite strain and calculates the elastic constants from the linear stress response.

Parameters:

configuration (BulkConfiguration.) – The BulkConfiguration for which the elastic constants should be calculated. optimizer (FIRE | LBFGS | None) – The optimizer method used for optimizing the internal coordinates after each strain. Set this to None to disable optimization of the strained cell. Default: LBFGS max_forces (PhysicalQuantity of type force) – The force threshold for the geometry optimization. Default: 0.005 * eV / Ang max_steps (int) – The maximum number of steps used by the optimizer. Default: 200 max_step_length (PhysicalQuantity of type length) – The maximum step length the optimizer may take. Default: 0.5 * Ang eta_max (float) – The maximum strain that can be be applied to the cell. Must be positive. Default: 0.002 n_eta (int) – The total number of different strain magnitudes for each strain vector. Must be larger than 1. Default: 3 enable_symmetry (bool) – Flag to select if only the independent elastic constants are calculated for each crystal system, or if all 21 constants are calculated. Default: True if the configuration has standard orientation, False otherwise. fit_order (int) – The order of the polynomial fit of the stress-strain dependency. Must be in the interval 1 to (n_eta - 1). Default: 1

bulkModulusHill()¶

Returns:	The Hill type bulk modulus.
Return type:	PhysicalQuantity of type pressure

bulkModulusReuss()¶

Returns:	The Reuss type bulk modulus.
Return type:	PhysicalQuantity of type pressure

bulkModulusVoigt()¶

Returns:	The Voigt type bulk modulus.
Return type:	PhysicalQuantity of type pressure

enableSymmetry()¶

Returns:	True if only the independent elastic constants are calculated for each crystal system, and False if all 21 constants are calculated
Return type:	bool

etaMax()¶

Returns:	The maximum strain that can be be applied to the cell.
Return type:	float

evaluate()¶

Returns:	The elastic constants as a shape (6, 6) array.
Return type:	PhysicalQuantity of type pressure

fitOrder()¶

Returns:	The order of the polynomial fit of the stress-strain dependency.
Return type:	int

maxForces()¶

Returns:	The force threshold for the geometry optimization.
Return type:	PhysicalQuantity of type force

metatext()¶

Returns:	The metatext of the object or None if no metatext is present.
Return type:	str | unicode | None

nEta()¶

Returns:	The total number of different strain magnitudes for each strain vector.
Return type:	int

nlprint(stream=None)¶

Print a string containing an ASCII table useful for plotting the AnalysisSpin object.

Parameters:	stream (python stream) – The stream the table should be written to. Default: NLPrintLogger()

poissonRatio()¶

Returns:	The Poisson ratios in various directions as a shape (3, 3) array.
Return type:	numpy.array

setMetatext(metatext)¶

Set a given metatext string on the object.

Parameters:	metatext (str | unicode | None) – The metatext string that should be set. A value of “None” can be given to remove the current metatext.

shearModulusHill()¶

Returns:	The Hill type shear modulus.
Return type:	PhysicalQuantity of type pressure

shearModulusReuss()¶

Returns:	The Reuss type shear modulus.
Return type:	PhysicalQuantity of type pressure

shearModulusVoigt()¶

Returns:	The Voigt type shear modulus.
Return type:	PhysicalQuantity of type pressure

youngsModulus()¶

Returns:	The Young’s moduli.
Return type:	PhysicalQuantity of type pressure

# Set up bulk configuration
bulk_configuration = BulkConfiguration(
    bravais_lattice=FaceCenteredCubic(5.4306*Angstrom),
    elements=[Silicon, Silicon],
    fractional_coordinates=[[ 0.  ,  0.  ,  0.  ],
                            [ 0.25,  0.25,  0.25]])

# Define the calculator
calculator = TremoloXCalculator(parameters=StillingerWeber_Si_1985())
bulk_configuration.setCalculator(calculator)
bulk_configuration.update()

# Calculate the elastic constants
elastic = ElasticConstants(bulk_configuration,
                           optimizer=LBFGS(),
                           max_forces=0.001*eV/Angstrom,
                           eta_max=0.002,
                           n_eta=3,
                           enable_symmetry=True,
                           fit_order=1)
# Print the results
nlprint(elastic)
# Save the elastic constants analysis object
nlsave('elastic_constants.nc', elastic)