qtcad.atoms.schrodinger module

class qtcad.atoms.schrodinger.Solver(atoms: Atoms, solver_params: SolverParams = None)

Bases: Solver

Atomistic tight-binding Schrödinger solver.

Attributes:

  • atoms (qtcad.atoms.atoms.Atoms) – Atomic structure.

  • tb_orbs (str) – Atomic orbital basis set used in the tight-binding model, which may either be “sp3” for an \(sp^3\) basis, “sp3s*” for an \(sp^3s^{\star}\) basis, or “sp3d5s*” for an \(sp^3d^5s^{\star}\) basis.

  • tb_spin (bool) – Whether a spin component is included in the tight-binding basis. If False, it is assumed the basis is composed of the atomic orbitals only. If True, it is assumed the basis is twice as large and is composed of the tensor products of the atomic orbitals with the spin up and spin down states. Set to False if and only if both atoms.soc and atoms.zeeman are False.

  • solver_params (qtcad.atoms.schrodinger.SolverParams) – Parameters of the atomistic tight-binding Schrödinger solver.

__init__(atoms: Atoms, solver_params: SolverParams = None) → None
Parameters:

  • atoms (qtcad.atoms.atoms.Atoms) – Atomic structure over which to solve the atomistic tight-binding Schrödinger equation.

  • solver_params (qtcad.atoms.schrodinger.SolverParams, optional) – Parameters to pass to the atomistic tight-binding Schrödinger solver. If None, parameters are set to reasonable defaults. Default: None.

get_hamiltonian() → bsr_matrix

Construct the full tight-binding Hamiltonian.

Returns:

Scipy.sparse.bsr_matrix) – \(\left(N N_B\right)\times\left(N N_B\right)\) matrix describing the tight-binding Hamiltonian of the atomic structure, where \(N\) is the number of atoms in the structure, where \(N_B = N_O N_S\) is the number of basis states per atom, where \(N_O\) is the number of atomic orbitals per atom (\(N_O = 4\) if tb_orbs is “sp3”, in which case the orbitals are \(\{s\), \(x\), \(y\), \(z\}\), \(N_O=5\) if tb_orbs is “sp3s*”, in which case the orbitals are \(\{s\), \(x\), \(y\), \(z\), \(s^{\star}\}\), and \(N_O=10\) if tb_orbs is “sp3d5s*”, in which case the orbitals are \(\{s\), \(x\), \(y\), \(z\), \(yz\), \(xz\), \(xy\), \(x^2-y^2\), \(3z^2-r^2\), \(s^{\star}\}\)), and where \(N_S\) is the number of spin states per atom (\(N_S = 1\) if tb_spin is False and \(N_S = 2\) if tb_spin is True). The matrix is partitioned into \(N_B\times N_B\) blocks in the \(\{s\), \(x\), \(y\), \(\dots \}\) basis if \(N_S = 1\) or in the \(\{s+\), \(s-\), \(x+\), \(x-\), \(y+\), \(y-\), \(\dots \}\) basis if \(N_S = 2\).

get_hamiltonian_soc() → bsr_matrix

Construct contribution to tight-binding Hamiltonian due to spin-orbit coupling.

Returns:

Scipy.sparse.bsr_matrix – \(\left(N N_B\right)\times\left(N N_B\right)\) matrix describing the contribution of spin-orbit coupling to the tight-binding Hamiltonian of the atomic structure, where \(N\) is the number of atoms in the structure and where \(N_B\) is the number of basis states per atom (\(N_B = 8\) if tb_orbs is “sp3”, in which case the basis states are \(\{s+\), \(s-\), \(x+\), \(x-\), \(y+\), \(y-\), \(z+\), \(z-\}\), \(N_B = 10\) if tb_orbs is “sp3s*”, in which case the basis states are \(\{s+\), \(s-\), \(x+\), \(x-\), \(y+\), \(y-\), \(z+\), \(z-\), \(s^{\star}+\), \(s^{\star}-\}\), and \(N_B = 20\) if tb_orbs is “sp3d5s*”, in which case the basis states are \(\{s+\), \(s-\), \(x+\), \(x-\), \(y+\), \(y-\), \(z+\), \(z-\), \(yz+\), \(yz-\), \(xz+\), \(xz-\), \(xy+\), \(xy-\), \((x^2-y^2)+\), \((x^2-y^2)-\), \((3z^2-r^2)+\), \((3z^2-r^2)-\), \(s^{\star}+\), \(s^{\star}-\}\); the matrix is partitioned into \(N_B\times N_B\) blocks.

solve(energy_target: float) → None

Solve the atomistic tight-binding Schrödinger equation.

The algorithm involves the following steps: (1) calculation of the external potential on the atomic positions, either by direct evaluation of self.atoms.phi or by interpolation from self.atoms.d, (2) construction of the tight-binding Hamiltonian matrix, and (3) partial diagonalization of this matrix. After execution, the eigenenergies and eigenfunctions are stored in self.atoms.energies and self.atoms.eigenfunctions.

Parameters:

energy_target (float) – The computed eigenpairs are those whose eigenenergies are closest to this energy.

class qtcad.atoms.schrodinger.SolverParams(inp_dict=None)

Bases: SolverParams

Parameters to pass to an atomistic tight-binding Schrödinger solver.

Attributes:

  • energy_shift_db (float) – Constant energy shift to be applied to the \(sp^3\) hybridized orbitals aligned with dangling bonds of surface atoms, emulating surface passivation. This suppresses unphysical surface states whose energies tend to lie within the bandgap. For more details, see Ref. [LOVAK04]. Values greater than 5.0*ct.e (5 eV) are recommended. Default: 9.0*ct.e (9 eV).

  • method (str) – Method used to interpolate the finite-element electric potential on the atomic position grid; options are “nearest” for nearest-neighbor interpolation, “linear” for linear interpolation, and “cubic” for cubic interpolation. Default: “nearest”.

  • num_states (int) – Number of eigenpairs to be computed. Default: 1.

  • noise (bool) – Whether a random Hermitian noise strategy is used to improve the numerical stability of the eigensolver. May be helpful in simulations with spin-orbit coupling, magnetic fields, and/or a rapidly-varying electric potential. Default: False.

  • maxiter (int) – Maximum allowed number of iterations for the eigensolver. If None, it is set to \(10 N N_B\), where \(N\) is the number of atoms in the structure and \(N_B\) is the number of basis states per atom.

  • rtol (float – Relative error on the eigenvalues setting the convergence criterion of the eigensolver. If 0, the eigensolver iterations run until the error on the eigenvalues reaches machine precision. Default: 0.

  • verbose (bool) – Whether information pertaining to the solver computation progression is printed. Default: False.

__init__(inp_dict=None) → None
Parameters:

inp_dict (dict, optional) – Dictionary specifying certain solver parameters to be used instead of their default values.