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

Using sequences of correlation consistent basis sets, complete basis s et (CBS) limits for the dissociation energy D-e of N-2 have been estim ated for a variety of commonly used electron correlation methods, Afte r extrapolation to the CBS limit, the difference between theory and ex periment 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 cont racted multireference configuration interaction (CMRCI), singles and d oubles coupled cluster theory with and without perturbative triple exc itations [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 c ontrast, for MP2 and MP4, D-e exhibits fortuitously good agreement wit h experiment for small basis sets but leads to CBS limits that are 11. 6 and 3.4 kcal/mol larger than experiment, respectively, Correlation o f 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 in crease still further.