INTRINSIC ERRORS IN SEVERAL AB-INITIO METHODS - THE DISSOCIATION-ENERGY OF N-2

Using sequences of correlation consistent basis sets, complete basis set (CBS) limits for the dissociation energy D-e of N-2 have been estimated for a variety of commonly used electron correlation methods, After extrapolation to the CBS limit, the difference between theory and experiment corresponds to the error intrinsic to the chosen theoretical method. Correlated wave functions (valence electrons correlated only) for which intrinsic errors have been estimated include internally contracted multireference configuration interaction (CMRCI), singles and doubles coupled cluster theory with and without perturbative triple excitations [CCSD, CCSD(T)], and second-, third-, and fourth-order Moller-Plesset perturbation theory (MP2, MP3, MP4), For CMRCI and CCSD(T), D-e converges smoothly from below the experimental value and yields the smallest intrinsic errors, -0.8 and -1.6 kcal/mol, respectively. In contrast, for MP2 and MP4, D-e exhibits fortuitously good agreement with experiment for small basis sets but leads to CBS limits that are 11.6 and 3.4 kcal/mol larger than experiment, respectively, Correlation of the Is core electrons is predicted to yield intrinsic errors of less than 1 kcal/mol for CMRCI and CCSD(T), while those for MP2 and MP4 increase still further.