Fig. 1. Convergence of the CP series of the dipole moment of H₂O calculated variationally (red) and nonvariationally (blue).

Errors (atomic units) plotted against the order of the perturbation expansion of the equilibrium dipole moment of H₂O, calculated variationally (red) and nonvariationally (blue). To the left, the coupled-cluster (CC) singles-and-doubles (CCSD) dipole moment is expanded about the CC singles (CCS) dipole moment; to the right, the CC singles-doubles-triples (CCSDT) dipole moment is expanded about the CCSD dipole moment. The wave functions that are used to calculate the red and blue curves are order by order the same; the only difference is how the dipole moment is calculated from these wave functions. All calculations are in the aug-cc-pVTZ basis, in the frozen-core approximation.