Partial combination of composite strategy and the B3LYP functional for the calculation of enthalpies of formation

The B3LYP functional was re-optimized using partially composite methods for the calculation of standard enthalpies of formation. Two initial methods (B3LYP-MCM1 and B3LYP-MCM2) differing in the type and number of optimized parameters were analyzed using B3LYP/6-31 + G(2df,p) as the reference energy. For the first method (B3LYP-MCM1), the exchange-correlation and higher-level correction parameters (HLC) were optimized and for the second method (B3LYP-MCM2), in addition to the previous parameters, scaling of the basis functions responsible for large errors in the enthalpy of formation were also optimized. The best parameters were also used as adapted functionals generating two other methods referred to as: B3LYP-MF1 and B3LYP-MF2. The best-performing results (B3LYP-MCM2 and B3LYP-MF2) presented mean absolute errors near 2.3 kcal mol^-1 for the G3/05 test set. This is a significant improvement when compared with the respective results from B3LYP/6-31 + G(2df,p), which yielded a mean absolute error of 5.3 kcal mol^-1. The errors were larger for B3LYP-MCM1 (4.17 kcal mol^-1) and B3LYP-MF1 (3.98 kcal mol^-1). The scaling of the experimental atomization energies used for the calculation of enthalpy of formation was also tested for all four methods. This empirical adjustment reduced the errors to 2 kcal mol^-1. The uncertainty of the best results with 95% confidence tended to be ± 5.5 kcal mol^-1. Substantial improvements were associated with the basis set adjustments.

Thermochemical Properties: Enthalpy, Entropy, and Heat Capacity of C2-C3 Fluorinated Aldehydes. Radicals and Fluorocarbon Group Additivity

Thermochemical properties of fluorinated aldehydes are important for understanding their stability and reactions in the environment and in thermal processes. Structures and thermochemical properties of C1 to C3 fluorinated aldehydes are determined by use of computational chemistry. Standard enthalpies of formation for 30 C₂- and C₃-fluorinated aldehydes and 31 radicals were calculated with 11 different ab initio and density functional theory methods: CBS-APNO, CBS-4M, CBS-QB3, M06-2X, ωB97X, B3LYP, G-2, G-3, G-4, and W1U via several series of isodesmic reactions. Entropy, S°₂₉₈, and heat capacities, C _p( T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31++G(d,p) density functional method. Potential barriers for the internal rotations are also from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potential curves. Literature data on standard enthalpies of formation of fluorinated aldehydes are compared. Thermochemical properties for the fluorinated carbon groups CO/C/F, C/CO/F3, C/CO/F/H2, C/C/CO/F/H, C/C/CO/F2, and C/C/CO/F/H are developed. Non-next nearest neighbor terms for the strong interactions resulting from fluorine atoms on adjacent and on second nearest carbon atoms are unfortunately, needed. The required non-next-neighbor interactions significantly reduce the practical application of group additivity for thermochemical properties of highly fluorinated halocarbons.

G4CEP: A G4 theory modification by including pseudopotential for molecules containing first-, second- and third-row representative elements

The G4CEP composite method was developed from the respective G4 all-electron version by considering the implementation of compact effective pseudopotential (CEP). The G3/05 test set was used as reference to benchmark the adaptation by treating in this work atoms and compounds from the first and second periods of the periodic table, as well as representative elements of the third period, comprising 440 thermochemical data. G4CEP has not reached a so high level of accuracy as the G4 all-electron theory. G4CEP presented a mean absolute error around 1.09 kcal mol(-1), while the original method presents a deviation corresponding to 0.83 kcal mol(-1). The similarity of the optimized molecular geometries between G4 and G4CEP indicates that the core-electron effects and basis set adjustments may be pointed out as a significant factor responsible for the large discrepancies between the pseudopotential results and the experimental data, or even that the all-electron calculations are more efficient either in its formulation or in the cancellation of errors. When the G4CEP mean absolute error (1.09 kcal mol(-1)) is compared to 1.29 kcal mol(-1) from G3CEP, it does not seem so efficient. However, while the G3CEP uncertainty is ±4.06 kcal mol(-1), the G4CEP deviation is ±2.72 kcal mol(-1). Therefore, the G4CEP theory is considerably more reliable than any previous combination of composite theory and pseudopotential, particularly for enthalpies of formation and electron affinities.