We have developed a space-restricted wave function (SRW) method for the analysis of various types of intramolecular interactions. In this study, we demonstrate the applicability of our SRW method to the analysis of the nature of the PO bond in phosphine oxide (R3PO), one of the hypervalent molecules. An interesting character of this bond has been extensively studied by focusing on the negative hyperconjugation of the O lone pair (nO) with the R3P group. We reinvestigated the nature of the bond in terms of a change in total energy to produce evidence for the validity of our method. The electronic states without the interaction involving three nO orbitals (R3P+−O−) produced by the method were used as reference states in the assessment of the effects of this nO–R3P interaction. The result confirms that this interaction plays an essential role in the nature of the bond and occurs between the nO orbitals and the PR antibonding orbitals, in agreement with previous studies. A molecular orbital (MO)-pair analysis technique shows that the nO–R3P interaction is decomposed into the negative hyperconjugation and the Pauli repulsion. Considering a reference state where the PO bond is completely broken (R3P2+···O2−) at an interacting distance, PO bond formation is attributed to one σ bond plus two 0.5 π bonds. This is equivalent to three banana bonds highly polarized to the O atom. Consequently, the SRW method suggested improved explanations of the nature of the PO bond. © 2012 Wiley Periodicals, Inc.