The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals

TLDR

The authors introduce two new hybrid meta exchange‑correlation functionals, M06 and M06‑2X, to expand the M06 suite. They parametrized M06 for transition metals and nonmetals and M06‑2X for nonmetals with high nonlocal exchange, then benchmarked the four M06 functionals against 12 others and Hartree–Fock on 403 energetic data from 29 diverse databases—including thermochemistry, kinetics, noncovalent interactions, transition‑metal bonding, and excitation energies—and also evaluated performance on bond lengths, vibrational frequencies, and zero‑point energies. The study recommends M06‑2X for main‑group thermochemistry, kinetics, noncovalent interactions, and electronic excitation energies, and M06 for organometallic/inorganometallic chemistry and noncovalent interactions.

Abstract

We present two new hybrid meta exchange- correlation functionals, called M06 and M06-2X. The M06 functional is parametrized including both transition metals and nonmetals, whereas the M06-2X functional is a high-nonlocality functional with double the amount of nonlocal exchange (2X), and it is parametrized only for nonmetals.The functionals, along with the previously published M06-L local functional and the M06-HF full-Hartree–Fock functionals, constitute the M06 suite of complementary functionals. We assess these four functionals by comparing their performance to that of 12 other functionals and Hartree–Fock theory for 403 energetic data in 29 diverse databases, including ten databases for thermochemistry, four databases for kinetics, eight databases for noncovalent interactions, three databases for transition metal bonding, one database for metal atom excitation energies, and three databases for molecular excitation energies. We also illustrate the performance of these 17 methods for three databases containing 40 bond lengths and for databases containing 38 vibrational frequencies and 15 vibrational zero point energies. We recommend the M06-2X functional for applications involving main-group thermochemistry, kinetics, noncovalent interactions, and electronic excitation energies to valence and Rydberg states. We recommend the M06 functional for application in organometallic and inorganometallic chemistry and for noncovalent interactions.