The utility of many-body decompositions for the accurate basis set extrapolation of ab initio data

We present a powerful new technique for the extrapolation of ab initio data based on many-body decompositions. Using the new methodology and subtle mo difications of the standard correlation consistent basis sets, the H+H-2 ba rrier height is estimated at 9.603 kcal/mol with a precision of about 0.003 kcal/mol; this extremely accurate result is all the more striking as it ca n be obtained using basis sets no larger than aug-cc-pVQZ. The method is al so used to yield highly accurate energies for the H+H-2 system on a grid of points previously calculated by quantum Monte Carlo. The three-body energy , summed with exact one- and two-body energies, is observed to yield a usef ul approximate lower bound for the total energy. The highly accurate energi es afforded by this method can also be used to assess the accuracy of previ ously calculated data that has been used to construct potential energy surf aces. As an example, we make a detailed comparison between the new results and the quantum Monte Carlo results for H+H-2. (C) 1999 American Institute of Physics. [S0021-9606(99)30633-4].