  Pythonic Black-box Electronic Structure Tool (PyBEST) represents a
fully-fledged modern electronic structure software package developed at
Nicolaus Copernicus University in Toru\'n. The package provides an efficient
and reliable platform for electronic structure calculations at the interface
between chemistry and physics using unique electronic structure methods,
analysis tools, and visualization. Examples are the (orbital-optimized)
pCCD-based models for ground- and excited-states electronic structure
calculations as well as the quantum entanglement analysis framework based on
the single-orbital entropy and orbital-pair mutual information. PyBEST is
written primarily in the Python3 programming language with additional parts
written in C++, which are interfaced using Pybind11, a lightweight header-only
library. By construction, PyBEST is easy to use, to code, and to interface with
other software packages. Moreover, its modularity allows us to conveniently
host additional Python packages and software libraries in future releases to
enhance its performance. The electronic structure methods available in PyBEST
are tested for the half-filled 1-D model Hamiltonian. The capability of PyBEST
to perform large-scale electronic structure calculations is demonstrated for
the model vitamin B12 compound. The investigated molecule is composed of 190
electrons and 777 orbitals for which an orbital optimization within pCCD and an
orbital entanglement and correlation analysis are performed for the first time.
