NOPT: Quantum chemistry package for multireference perturbation theory calculations and its non-orthogonal extensions.

NOPT is an ab initio package for calculations using Non-Orthogonal methods and multireference Perturbation Theory. It can be used both as a ready-to-use quantum chemistry program and as an open-source system for modifying existing methods and developing new ones. The key functionality includes restricted Hartree-Fock, configuration interaction with single excitations, complete active space self-consistent field, and several multireference perturbation theory (MRPT) methods. The MRPT module provides extended multiconfigurational quasi-degenerate perturbation theory (XMCQDPT2), multipartitioning perturbation theory (MPPT2), and a novel variant of uncontracted MRPT-CDAS-PT2. NOPT allows one to perform a complete computational workflow starting from a molecular geometry. It also contains an interface for using data calculated by other quantum chemistry programs. The implemented non-orthogonal methods are aimed at the fragment-based calculations with multireference wave functions. All the methods are implemented with the resolution of identity approximation to gain higher performance. We hope that this makes NOPT a useful tool for the calculations with both standard and novel methods of quantum chemistry.