A complete basis set model chemistry. VI. Use of density functional geometries and frequencies

Abstract

The recently introduced complete basis set, CBS-Q, model chemistry is modified to use B3LYP hybrid density functional geometries and frequencies, which give both improved reliability (maximum error for the G2 test set reduced from 3.9 to 2.8 kcal/mol) and increased accuracy (mean absolute error reduced from 0.98 to 0.87 kcal/mol), with little penalty in computational speed. The use of a common method for geometries and frequencies makes the modified model applicable to transition states for chemical reactions.

Kinetic Stability and Propellant Performance of Green Energetic Materials

“…[42,43,[45][46][47] The mean absolute deviation in the G2 test set is reported to be 0.87 kcal mol À1 . [42] The accuracy in predicting kinetics of pericyclic reactions is most important to our study. Guner et al evaluated this for 11 different reactions [46,47] and CBS-QB3 was found to have a mean absolute deviation value of 1.9 and a standard deviation of 1.6 kcal mol À1 , when compared to experimentally determined activation enthalpies.…”

“…The highly accurate multicomponent complete basis set (CBS-QB3) [42,43] method of Montgomery, Ochterski, and Peterson was used for obtaining energetics in the gas phase, using the Gaussian 03 [44] program. CBS-QB3 uses coupled cluster (CCSD(T)) energies, which are extrapolated to the basis-set limit using MP2 and MP4 energies and empirical corrections.…”

“…Geometries and thermodynamic corrections were obtained at the B3LYP/6-311GA C H T U N G T R E N N U N G (d,p) level. [42,43] CBS-QB3 has been extensively tested in several benchmarks and is expected to produce reliable results. [42,43,[45][46][47] The mean absolute deviation in the G2 test set is reported to be 0.87 kcal mol À1 .…”

“…[42,43] CBS-QB3 has been extensively tested in several benchmarks and is expected to produce reliable results. [42,43,[45][46][47] The mean absolute deviation in the G2 test set is reported to be 0.87 kcal mol À1 . [42] The accuracy in predicting kinetics of pericyclic reactions is most important to our study.…”

Conformational Flexibility of l-Alanine Zwitterion Determines Shapes of Raman and Raman Optical Activity Spectral Bands

“…This is a general problem with ab initio Hessians of finite accuracy and has been described previously. 44 The one-dimensional energy dependencies plotted in Figure 2 provide shallow potential wells/plateaus for the minima. For the rotational barrier of CO 2 -the computations seem to converge to the value of about 4 kcal/mol obtained for the best basis set (6-31++G**), while for NH 3 + the barrier is lower and practically disappears for the best basis.…”