Achieving Near-Chemical Accuracy with Atom-Centered Potential-Augmented M06-2X

The calculation of accurate thermodynamic properties, such as activation and reaction free energies, is an essential tool for elucidating reaction mechanisms. The method most commonly employed, density functional theory (DFT), generally falls short of chemical accuracy (1 kcal/mol error), resulting in order-of-magnitude errors in the calculation of equilibrium constants and rate constants. The inaccuracy of common exchange-correlation functionals is compounded by the use of small and medium-sized basis sets, which are mandatory for molecular systems with realistic sizes. The basis set incompleteness error incurred by the use of these basis sets further diminishes the accuracy of thermochemical calculations. In previous works, we proposed energy corrections based on the use of atom-centered potentials (ACP) as a way of mitigating functional and basis set incompleteness error. ACPs are one-electron potentials that, when applied in combination with a functional and basis set, yield results comparable to a higher level of theory with little additional cost. In this work, and based on previous studies, we develop ACPs for thermochemical calculations based on the M06-2X density functional and the 6-31+G(d,p) basis. The resulting M06-2X/6-31+G(d,p)-ACP method is tested on a variety of cases, demonstrating excellent performance for thermochemistry and kinetics calculations well outside the parametrization set, particularly for reaction energies and barrier heights of moderately large systems. At the same time, M06-2X/6-31+G(d,p)-ACP retains the low cost of the uncorrected functional and basis set combination.