H. Muller et al., Toward spectroscopic accuracy of ab initio calculations of vibrational frequencies and related quantities: a case study of the HF molecule, THEOR CH AC, 100(1-4), 1998, pp. 85-102
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.