Complete basis set correlation energies. IV. The total correlation energy of the water molecule

The second-order and total correlation energy of the water molecule are calculated with a basis set of 45 atomic pair natural orbitals. The asymptotic forms for the convergence of pair natural orbital expansions are then used to extrapolate to the values of the pair energies and pair-coupling terms that would be obtained with a complete basis set. The extrapolated complete basis set second-order correlation energy is equal to −361.1 mhartree and the extrapolated complete basis set total correlation energy is equal to −370.6 mhartree in agreement with the experimental value (−370±3 mhartree). The error in the extrapolated total correlation energy is more than an order-of-magnitude smaller than the error in any previous calculation.