G3-RAD and G3X-RAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) procedures for radical thermochemistry
- 15 March 2003
- journal article
- Published by AIP Publishing in The Journal of Chemical Physics
- Vol. 118 (11), 4849-4860
- https://doi.org/10.1063/1.1544731
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 respectively, compared with 4.26 and for standard G3 and G3X. G3-RAD and G3X-RAD successfully predict heats of formation with MADs of 3.68 and respectively (compared with 3.93 and for standard G3 and G3X), and perform particularly well for radicals with MADs of 2.59 and respectively (compared with 3.51 and 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 respectively, compared with 5.44 and 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 respectively, versus 4.94 and G3(MP2)-RAD shows similar performance to G3(MP2) for radical heats of formation (MAD=5.10 versus while G3X(MP2)-RAD performs significantly better than G3X(MP2) (MAD=4.67 versus
Keywords
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