The performance of multi-reference ACPF-like methods for the dipole moment of FeO

We compare the performance of the multi-reference formulations of averaged coupled-pair functional (ACPF), averaged quadratic coupled-cluster (AQCC), and the novel ACPF-2, together with configuration interaction, CI(SD), on the difficult case of the dipole moment of FeO. AQCC and ACPF-2 give the same results and the stability of these two methods is dramatically better than that of ACPF. Since ACPF-2 is generally close to ACPF when the zeroth-order wavefunction, Ψ0, is good approximation to the total wavefunction, ACPF-2 combines the favorable features of AQCC (stability) and of ACPF (accuracy) without having the drawbacks of these latter two methods. Thus, regardless on the quality of Ψ0, the novel ACPF-2 is the method to be preferred among the ACPF-like functionals.