Subchemical accuracy in combustion thermochemistry via ab initio methods: from basis set extrapolations to explicitly-correlated wavefunctions

Wavefunction-based methods in quantum chemistry provide simultaneously the most accurate, but also most expensive method for computing molecular properties. In particular, the realm of ``subchemical' accuracy better than 1.0 kcal/mol has now become feasible even for difficult combustion intermediates. However, accurate methods must treat electron correlation, leading to problems which explode in cost as N^6 or worse in the number of atoms. More compact representations of the wavefunction are necessary to further increase the scope and accuracy. Explicitly-correlated methods have recently emerged which mathematically represent the wavefunction directly in the interelectronic distance r12, providing a much more natural representation of the electron correlation. Herein we develop a series of explicitly-correlated (R12) methods suitable for open-shell radicals in combustion problems. These R12 methods are compared to existing methodologies such as the focal point approach (FPA), demonstrating their utility in combustion thermochemistry.