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 of realistic size. In previous works, we proposed energy corrections based atom-centered potentials (ACPs) to mitigate functional and basis set incompleteness error. ACPs are one-electron potentials that, when applied with a functional and basis set combination, yield results comparable to a higher level of theory with little or no additional computational cost.

In this work, we developed ACPs for thermochemical calculations using M06-2X/6-31+G(d,p). 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 system. At the same, time M06-2X/6-31+G(d,p)-ACP retains the low cost of the uncorrected functional and basis set combination.