Quantum operators

qOperator represents quantum-number-resolved operator blocks in a compact, symmetry-aware form. It is used throughout the NRG implementation to manage block structure, basis transformations, and sector-dependent operator data.

How operator blocks are represented

Internally, a qOperator is a map from a pair of sector indices (i, j) to a qmatrix. Each stored matrix contains the operator elements connecting states in sector j to states in sector i. Blocks that are absent from the map are treated as structurally zero; get(i, j) reports an optional pointer so callers can distinguish a missing block without allocating a dense matrix for it.

The set overloads insert a block by copying or moving its matrix and reject duplicate sector pairs. unitaryTransform rotates each stored block with the corresponding sector eigenvector matrices, applying the left-sector adjoint and right-sector rotation. This lets NRG code update an operator after diagonalizing the Hamiltonian without expanding it into the full Hilbert space. clear removes all blocks, and display prints them for inspection.

For the generated class reference, see the qOperator class documentation.