The accuracy of atomization energies from explicitly correlated coupled-cluster calculations

The accuracy of atomization energies obtained from explicitly correlated co upled-cluster R12 calculations (CC-R12)-including single and double excitat ion operators (CCSD-R12) and a posteriori perturbative corrections for trip le excitations [CCSD[T]-R12 and CCSD(T)-R12]-is studied for CH2((1)A(1)), N H3, H2O, HF, N-2, CO, and F-2. The basis-set convergence with functions of high angular momentum is demonstrated. Unlike for conventional calculations , already the spdf saturation on nonhydrogen atoms and spd saturation on hy drogen are sufficient for CC-R12 calculations to provide results accurate t o within 1 kJ/mol of the limit of a complete basis. Remaining small uncerta inties at the CCSD[T]-R12 or CCSD(T)-R12 levels are attributed to the insuf ficient convergence within the coupled-cluster hierarchy towards the limit of full configuration interaction. It is shown that near the basis-set limi t (as provided by CC-R12 calculations) the CCSD[T] variant of the triples c orrection gives, on average, results closer to the experimental data than i ts CCSD(T) counterpart. Approximate error bars are estimated by one single CC-R12 calculation from the difference between the CCSD[T] and CCSD(T) meth ods and from the second-order electronic cusp correction in standard approx imation B. (C) 2001 American Institute of Physics.