The basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mo/ller–Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mEh by means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances rij. For the extrapolations to the basis-set limit of the correlation energies, fits of the form a+bX−3 (where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported.

Articles you may be interested inA new parametrizable model of molecular electronic structure J. Chem. Phys. 135, 134120 (2011); 10.1063/1.3646498 Equation-of-motion coupled cluster method with full inclusion of connected triple excitations for electronattached states: EA-EOM-CCSDTThe full coupled-cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp( T J + T2 + T 3 ) I <1>0) is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C 2 H 2 , CO, Ne, F-, and H 2 0 to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcallmol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth-order MBPT are reported.