Coupled-cluster methods with internal and semi-internal triply excited clusters: Vibrational spectrum of the HF molecule

The recently proposed, single-reference, coupled-cluster (CC) methods with singly, doubly, and triply excited clusters, in which triexcited clusters T -3 are restricted to internal and semi-internal components defined through active orbitals, designated as the CCSD{t(')} and CCSDt approaches [P. Piec uch, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 110, 6103 (1999)] have been used to obtain the potential energy function and the vibrational spectrum of the HF molecule, as described by the large, aug-cc-pvtz, basis set. A comparison has been made with the vibrational term values obtained a t the very high, full CCSDT (CC singles, doubles, and triples), level and w ith the experimental (RKR) data. In spite of using the restricted Hartree-F ock reference, the calculated CCSD{t'} and CCSDt vibrational term values ha ve been found to be in much better agreement with the full CCSDT and RKR da ta than the results of conventional CCSD (CC singles and doubles) calculati ons, which produce errors of an order of 2000 cm(-1) for high-lying states near dissociation. It has been demonstrated that the CCSDt approach is capa ble of providing the vibrational spectrum of the full CCSDT quality. (C) 19 99 American Institute of Physics. [S0021-9606(99)30239-7].