Can ordinary single-reference coupled-cluster methods describe potential energy surfaces with nearly spectroscopic accuracy? The renormalized coupled-cluster study of the vibrational spectrum of HF

The recently proposed renormalized (R) and completely renormalized (CR) CCS D(T) and CCSD(TQ) methods, which remove the failing of the standard CCSD(T) and CCSD(TQ(f)) approaches at large internuclear separations, have been us ed to obtain the potential energy function and the vibrational spectrum of the HF molecule. The vibrational term values obtained in the renormalized a nd completely renormalized CCSD(T) and CCSD(TQ) calculations have been foun d to be in a better agreement with the experimental [Rydberg-Klein-Rees (RK R)] data than than the results of the expensive full CCSDT calculations. Th e simple R-CCSD(T) method gives < 10 cm(-1) errors for the vibrational ener gies up to similar to 41 000 cm(-1). The CR-CCSD(T) and CR-CCSD(TQ) methods reduce the similar to 300 cm(-1) errors in the full CCSDT results for the high-lying states near dissociation to 100-200 cm(-1). (C) 2001 American In stitute of Physics.