pengWann: Descriptors of chemical bonding from Wannier functions

First-principles calculations of periodic systems typically represent electronic structures through sets of eigenvectors and their corresponding eigenvalues, obtained by diagonalising a many-body Hamiltonian. These eigenvectors, known as Bloch states, molecular orbitals, or crystal orbitals, are generally delocalised across the entire structure, making them difficult to interpret in terms of chemically intuitive concepts such as bonds. To address this, it is common practice to project these extended Bloch states onto a localised basis, enabling the calculation of various local descriptors of chemical bonding or electronic structure. pengwann is a Python package for calculating some of these descriptors by projecting Bloch states onto Wannier functions (Kundu et al., 2021). Wannier functions provide a highly optimised local basis and, when derived from energetically isolated bands, span the same Hilbert space as the canonical Bloch states. The package provides a simple interface to the popular Wannier90 code (Pizzi et al., 2020), making it readily accessible to researchers already familiar with this widely-used tool.