ACCURACY IN ABINITIO REACTION-ENERGY COMPUTATIONS .1. COMPOUNDS OF 1ST-ROW ELEMENTS

Ab initio enthalpy computations were carried out for over 40 gas-phase diamagnetic molecules (including 18 hydrocarbons). All employed optimized geometries, basis sets ranging from 4-31G to 6-311++G(2df,2pd), and a series of electron-correlation approximations (MP2, MP3, MP4SDQ, and MP4SDTQ, as well as CCD, CCSD, CCSD+T(CCSD), and several CCSDT versions). The energies of forming the various molecules from the nuclei and electrons at O K with no nuclear motion were calculated from experimental data and compared with the various ab initio values. The percentage difference between these experimental and ab initio values without correlation was found to be characteristic of each molecule regardless of the size of the basis set. For the hydrocarbons (and hydrides of other first-row atoms) these differences could be quantitatively related to the ratio of the number of hydrogens to the other atoms in each molecule. Electron correlation reduced this percentage difference by roughly a factor of 2. The enthalpies at 298 K of chemical reactions between the molecules were considered in terms of the disagreement between the experimental and theoretical enthalpies, with emphasis on generic classes of reactions, e.g., formation reactions involving (1) dehydrogenation of the common hydrides, or (2) combination of the homonuclear diatomics. Generic reactions showed up regularities in disagreements between experiment and theory. Reasons for occasional large disagreements were probed. © 1990 American Chemical Society.