G3-RAD and G3X-RAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) procedures for radical thermochemistry

TLDR

The authors developed the G3‑RAD, G3X‑RAD, G3(MP2)‑RAD, and G3X(MP2)‑RAD procedures to provide reliable thermochemistry for free radicals and evaluated them on the G2/97 test set. These procedures employ B3‑LYP geometries and scaled vibrational frequencies to reproduce ZPVEs, use URCCSD(T) as the highest‑level correlation method, and replace UMP with RMP to approximate basis‑set extension effects. Across the G2/97 set, the RAD variants achieved lower mean absolute deviations than standard G3/G3X, with G3X‑RAD attaining MADs of 3.65 kJ mol⁻¹ overall and 2.50 kJ mol⁻¹ for radical heats, while G3(MP2)‑RAD and G3X(MP2)‑RAD also improved upon their standard counterparts.

Abstract

The G3-RAD, G3X-RAD, G3(MP2)-RAD, and G3X(MP2)-RAD, procedures, designed particularly for the prediction of reliable thermochemistry for free radicals, are formulated and their performance assessed using the G2/97 test set. The principal features of the RAD procedures include (a) the use of B3-LYP geometries and vibrational frequencies (in place of UHF and UMP2), including the scaling of vibrational frequencies so as to reproduce ZPVEs, (b) the use of URCCSD(T) [in place of UQCISD(T)] as the highest-level correlation procedure, and (c) the use of RMP (in place of UMP) to approximate basis-set-extension effects. G3-RAD and G3X-RAD are found to perform well overall with mean absolute deviations (MADs) from experiment of 3.96 and 3.65 kJ mol−1, respectively, compared with 4.26 and 4.02 kJ mol−1 for standard G3 and G3X. G3-RAD and G3X-RAD successfully predict heats of formation with MADs of 3.68 and 3.11 kJ mol−1, respectively (compared with 3.93 and 3.60 kJ mol−1 for standard G3 and G3X), and perform particularly well for radicals with MADs of 2.59 and 2.50 kJ mol−1, respectively (compared with 3.51 and 3.18 kJ mol−1 for standard G3 and G3X). The G3(MP2)-RAD and G3X(MP2)-RAD procedures give acceptable overall performance with mean absolute deviations from experiment of 5.17 and 4.92 kJ mol−1, respectively, compared with 5.44 and 5.23 kJ mol−1 for standard G3(MP2) and G3X(MP2). G3(MP2)-RAD and G3X(MP2)-RAD give improved performance over their standard counterparts for heats of formation (MADs=4.73 and 4.44 kJ mol−1, respectively, versus 4.94 and 4.64 kJ mol−1). G3(MP2)-RAD shows similar performance to G3(MP2) for radical heats of formation (MAD=5.10 versus 5.15 kJ mol−1) while G3X(MP2)-RAD performs significantly better than G3X(MP2) (MAD=4.67 versus 5.19 kJ mol−1).