NAST 2.0: Magnetic Field Effects and Marcus-Levich-Jortner Theory for Complex Open-Shell Systems.

We present a new version of the nast software package for predicting the kinetics of unimolecular spin-dependent processes, including intersystem crossings, spin-forbidden reactions, and spin crossovers with nonadiabatic statistical theory. With substantial extensions and upgrades, the new nast 2.0 release enables the calculations of (1) rate constants for spin-dependent reactions in an external magnetic field, (2) Marcus-Levich-Jortner rate constants with a new mlj program for complex open-shell systems, (3) Huang-Rhys factors and reorganization energies with a new vib tool, (4) Wigner and Eckart quantum tunneling correction coefficients, (5) minimum energy crossing points with a new pymecp program interfaced with the orca electronic structure package, (6) effective Hessian with the q-chem and orca packages, (7) intrinsic reaction coordinate fits with the orca package for more advanced nast calculations with the Zhu-Nakamura transition probabilities, and (8) transition probabilities and rate constants with integration step-size control. Together, these capabilities significantly enhance the applicability of nast for studying the kinetics of spin-dependent processes even in complex open-shell systems. This paper describes the objectives and applications of the new capabilities illustrated with numerical examples. nast is an open-source software package and is available free of charge.