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G3-RAD and G3X-RAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) Procedures for Radical Thermochemistry
David J. Henry, Michael B. Sullivan, and Leo Radom
J. Phys. Chem., 2003, 118, 4849-4860.
Contribution from Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
Received November 4, 2002
Accepted December 12, 2002
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-1or 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 or 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-1for 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, espectively, compared with 5.44 and 5.23 kJ mol-1or 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).