An investigation of the two-electron chemical bond calculating energy expectation values from density matrix partitioning. I

Bond energy contributions are calculated by partitioning the first and second order density matrices of two-electron homonuclear molecules using Ruedenberg's procedure of chemical bond analysis. The density matrices are obtained from valence bond wavefunctions including ionic terms with scaled (and floated) Slater type orbitals. The systems are selected for investigating different bond phenomena (usual bond: H2, metastable state: He22+, three-center bond: H3+). Expectation values for kinetic and potential operators are calculated with different terms from the density matrices. The results are given in the form of energy plots of the electronic ground states. The actual bonding is explained by an energetic compromise between promotion, interference, and quasiclassical effects the importance of which varies for each molecule. (Auth.)