Energy of theH2O Molecule Calculated by Many-Body Perturbation Theory

Many-body perturbation theory is used to calculate a value of (- 76.48±0.07) a. u. for the total energy of the ${\mathrm{H}}_2$O molecule. This value is obtained with a basis set of bound and continuum orbitals of angular momentum $s$, $p$, $d$, $f$, and $g$, computed in a potential appropriate to neutral oxygen. Diagrams involving the net interaction with passive unexcited states, minus the single-particle potential, and the hydrogen nuclei are evaluated through third order, and estimates of higher-order diagrams are made. Both pair and three-body correlation-energy diagrams are included in this calculation.