A comparison of two approaches to perturbation triple excitation corrections to the coupled-cluster singles and doubles method for high-spin open-shell systems

An analysis of molecular properties is presented for several first-row diatomic molecules as determined at the CCSD(T) level of theory. In particular, a comparison of spectroscopic constants predicted by the ROHF-CCSD(T) methods of Scuseria vs Gauss et al. is given. Accurate determination of the values of equilibrium bond lengths, dissociation energies, harmonic vibrational frequencies, anharmonic constants, vibration-rotation coupling constants, and centrifugal distortion constants show only small differences between the two methods. In particular, we show that the average absolute differences between the two methods for the equilibrium bond length, harmonic vibrational frequency, and the dissociation energy are 0.000 09 Angstrom, 0.8 cm(-1) and 0.1 kcal/mol, respectively. (C) 1996 American Institute of Physics.