Quiqbox: a highly customizable basis set generator

Quantum and classical computers are being applied to solve ab initio problems in physics and chemistry. In the NISQ era, solving the "electronic structure problem" has become one of the major benchmarks for identifying the boundary between classical and quantum computational power. Electronic structure in condensed matter physics is often defined on a lattice grid while electronic structure methods in quantum chemistry rely on atom-centered single-particle basis functions. Grid-based methods require a large number of single-particle basis functions to obtain sufficient resolution when expanding the N-body wave function. Typically, fewer atomic orbitals are needed than grid points but the convergence to the continuum limit is less systematic. To investigate the consequences and compromises of the single-particle basis set selection on electronic structure methods, we need more flexibility than is offered in standard solid-state and molecular electronic structure packages. Thus, we have developed an open-source software tool called "Quiqbox" in the Julia programming language that allows for easy construction of highly customized floating basis sets. This package allows for versatile configurations of single-particle basis functions as well as variational optimization based on automatic differentiation of basis set parameters. Quiqbox also provides interfaces for calculating one- and two-body integrals over the custom basis sets using external libraries. Exporting to Molden file format is also supported. Some applications of this software package will be considered in the context of NISQ quantum devices.