Pythonic Black-box Electronic Structure Tool (PyBEST). An open-source Python platform for electronic structure calculations at the interface between chemistry and physics

Abstract

Pythonic Black-box Electronic Structure Tool (PyBEST) represents a fully-fledged modern electronic structure software package developed at Nicolaus Copernicus University in Toruń. 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 Python 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. Program summaryProgram title: PyBESTCPC Library link to program files:https://doi.org/10.17632/xf9kb7yfwr.1Developer’s repository link:https://zenodo.org/record/3925278#.X5KAZS8Rq6sLicensing provisions: GNU General Public License 3Programming language:Python, C＋＋Nature of problem: Efficient and reliable modeling of electronic structures featuring both weakly- and strongly-correlated electrons. Small- and large-scale quantum-mechanical problems at the interface between chemistry and physics comprising both quantum chemical and model Hamiltonians. Specifically, modeling potential energy surfaces of complex electronic structures including bond breaking/formation, elucidating complex electronic structures through the picture of interacting orbitals, describing noncovalent interactions, ultra-cold trapped quantum gases, and a variety of applications in interdisciplinary quantum mechanical-based problems.Solution method: Modular implementation of a series of unconventional (and conventional) electronic structure models based on the pCCD ansatz to solve the electronic Schrödinger equation. These include the description of both ground- and excited-states, the determination of interaction energies, and the analysis and interpretation of electronic wavefunctions. All modules are implemented in the modern Python programming language, where bottleneck operations are handled by C＋＋ code interfaced by the Pybind11 header-only library. The implemented (wavefunction) modules and modular code structure make PyBEST a very efficient alternative to existing electronic structure packages.Additional comments including restrictions and unusual features:PyBEST features unconventional electronic structure methods (pCCD and post-pCCD methods) that are not available in any other quantum chemistry/physics software package. It also includes a general orbital entanglement and correlation module that supports both pCCD and selected post-pCCD methods. PyBEST is designed to be easy to use and code in. Due to its modularity (for instance of the tensor contraction engine), new Python modules and features can be straightforwardly imported and exploited without changing any wavefunction modules directly.