Perturbation theory of the electron correlation effects for atomic and molecular properties. VII. Complete fourth-order MBPT study of the dipole moment and dipole polarizability of H2O

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Abstract
The dipole moment and dipole polarizability tensor of the water molecule are calculated by using a complete fourth‐order many‐body perturbation theory (MBPT) approach based on the coupled Hartree‐Fock (CHF) solution for the one‐electron perturbation problem. The CHF‐based MBPT calculations reveal that the fourth‐order correlation corrections involving triple and single substitutions in the reference HF determinant are by no means negligible. On the contrary they represent the two largest in magnitude fourth‐order terms. The present study reveals that good results obtained with several approximate fourth‐order MBPT schemes follow from some mutual cancellations between the neglected lower and higher‐order contributions. Among these approximate MBPT approaches particular attention is given to the CHF‐based MBPT treatment which involves only singly and doubly substituted states (SD‐MBPT) and which provides a reasonable agreement with both the experimental data and more accurate theoretical values.