# FixedSpin¶

class FixedSpin(spin_directions=None, fixed_spin_energy=None)

Class for representing a fixed spin configuration.

The spins are fixed by adding an energy penalty on given atoms if their spins are not aligned with the desired direction.

Parameters: spin_directions (list of tuple of (int | PeriodicTableElement, PhysicalQuantity of type angle, PhysicalQuantity of type angle)) – The fixed spin direction for each atom should be given as a tuple with three numbers, corresponding to the atom index and the spherical coordinates theta and phi defining the direction. Alternatively, the spin directions can also be given by element. Default: (index, 0.0 * Degrees, 0.0 * Degrees) for each atom fixed_spins_energy (PhysicalQuantity of type energy) – Scaling of the energy penalty if the spins are not aligned with the fixed spin direction. Default: 5.0 * eV
fixedSpinEnergy()
Returns: The fixed spin energy PhysicalQuantity of type energy

## Usage Examples¶

Setup a FixedSpin object using atom indices:

fixed_spin = FixedSpin(spin_directions=[(0, 60*Degrees, 30*Degrees),
(2, -45*Degrees, 330*Degrees)])


Setup a FixedSpin object by element:

fixed_spin = FixedSpin(spin_directions=[(Hydrogen, 90*Degrees, 30*Degrees),
(Carbon, 54*Degrees, 300*Degrees)])


The following example demonstrates how to run a calculation with the FixedSpin object for a chromium dimer. Note that the chosen parameters might not necessarily represent a physically relevant calculation.

Warning

If FixedSpin is used in conjunction with InitialSpin, the latter should define collinear spin directions for all atoms in the system.

 # Define the elements and coordinates.
elements = [Chromium, Chromium]
cartesian_coordinates = [[0.0, 0.0, 0.0],
[0.0, 0.0, 1.77]]*Angstrom

# Set up the Cr dimer configuration.
configuration = MoleculeConfiguration(
elements=elements,
cartesian_coordinates=cartesian_coordinates
)

# Set up an LCAO calculator with non-collinear spin and
# the LDA exchange-correlation.
calculator = LCAOCalculator(
exchange_correlation=NCLDA.PZ,
numerical_accuracy_parameters=NumericalAccuracyParameters(
density_mesh_cutoff=75.0*Hartree,
occupation_method=FermiDirac(300.0*Kelvin),
)
)

# Set an initial spin of 1.0 in the direction (theta, phi) = (0, 0).
spin_list = [
(0, 1.0, 0*Degrees, 0*Degrees),
(1, 1.0, 0*Degrees, 0*Degrees)
]
initial_spin = InitialSpin(scaled_spins=spin_list)

# Set the spin to be fixed in the direction (theta_0, phi_0) = (0, 0)
# for the first atom and in the direction (theta_1, phi_1) = (15 Degrees, 0)
# for the second atom.
theta_0_fixed = 0*Degrees
theta_1_fixed = 15*Degrees
fixed_spin_list = [(0, theta_0_fixed, 0*Degrees), (1, theta_1_fixed, 0*Degrees)]
fixed_spin = FixedSpin(spin_directions=fixed_spin_list, fixed_spin_energy=5*eV)

# Set the fixed spins as a local magnetic field.
configuration.setMagneticField(fixed_spin)

# Set the calculator on the configuration and run the calculation.
configuration.setCalculator(calculator, initial_spin=initial_spin)
configuration.update()