Spin-half impurity model

The spinhalf model is the canonical single-impurity fermionic model used for spinful Anderson-type and Kondo-type calculations. It defines the local Hilbert space, quantum-number sectors, and the fermionic operators needed by the NRG solver.

How the local model is built

The constructor creates a two-orbital fermionBasis for spin up and spin down, giving the four local occupation states. It forms the number operators from each creation operator and its adjoint, then assembles the local Hamiltonian

\[H = \epsilon_d(n_\uparrow + n_\downarrow) + h(n_\uparrow - n_\downarrow) + U n_\uparrow n_\downarrow.\]

The Hamiltonian is converted to quantum-number blocks and diagonalized sector by sector. The model exposes the resulting sector labels and energies, the spin-resolved creation operators, and the fermion-parity factor chi_Q used when operators from neighboring sites are combined. It also constructs the double-occupancy operator n_up n_down for correlation and response calculations.

spinhalf describes a single local site only: it does not add Wilson bath sites or perform NRG truncation. Those responsibilities belong to nrgcore, which consumes the model’s basis, energies, and operators.

For the detailed API and member list, see the spinhalf class documentation.