Bond length, dipole moment, and harmonic frequency of CO

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Abstract
A detailed comparison of some properties of CO is given, at the modified coupled‐pair functional, single and double excitation coupled‐cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one‐particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h]+(1s 1p 1d) basis set, the bond distance is about 0.005a 0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm−1 too small, when compared with experimental results.