Valence: A Massively Parallel Implementation of the Variational Subspace Valence Bond Method

This work describes the software package, Valence, for the calculation of molecular<br>energies using the variational subspace valence bond (VSVB) method. VSVB is a highly scalable ab initio electronic structure method based on non-orthogonal orbitals. Important features of practical value include: Valence bond wave functions of Hartree–Fock quality can be constructed with a single determinant; excited states can be modeled with a single configuration or determinant; wave functions can be constructed automatically by combining orbitals from previous calculations. The opensource software package includes tools to generate wave functions, a database of generic orbitals, example input files, and a library build intended for integration with other packages. We also describe the interface to an external software package, enabling the computation of optimized molecular geometries and vibrational frequencies.