Toward spectroscopic accuracy of ab initio calculations of vibrational frequencies and related quantities: a case study of the HF molecule

Calculations at various coupled-cluster (CC) levels with and without the in clusion of linear r(ij)-dependent terms are performed for the HF molecule i n its ground state with a systematic variation of basis sets. The main emph asis is on spectroscopic properties such as the equilibrium distance r(e) a nd the harmonic vibration frequency omega(e). Especially with the R12 metho ds (including linear r(ij)-dependent terms), convergence to the basis set l imit is reached. However, the results (at the basis set limit) are rather s ensitive to the level of the treatment of electron correlation. The best re sults are found for the CCSDT1-R12 and CCSD[T]-R12 methods (CCSD[T] was pre viously called CCSD + T(CCSD)), while CCSD(T) overestimates omega(e) by app roximate to 6 cm(-1). The good agreement of conventional CCSD(T) with exper iment for basis sets far from saturation (e.g. truncated at g-functions) is probably the result of a compensation of errors. The contribution of core- correlation is nonnegligible and must be included (effect on omega(e) appro ximate to 5 cm(-1)). Relativistic effects are also important (2-3 cm(-1)), while adiabatic effects are much smaller (< 1cm(-1)) and nonadiabatic effec ts on omega(e) can be simulated in replacing nuclear by atomic masses; for rotation nuclear masses appear to be the better choice, at least for hydrid es. From a potential curve based on calculations with the CCSDT1-R12 method with relativistic corrections, the IR spectrum is computed quantum-mechani cally. Both the band heads and the rotational structures of the observed sp ectra are reproduced with a relative error of approximate to 10(-4) for the three isotopomers HF, DF, and TF.