A simple natural orbital mechanism of "pure" van der Waals interaction in the lowest excited triplet state of the hydrogen molecule

O.V. Gritsenko, E.J. Baerends

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A treatment of van der Waals (vdW) interaction by density-matrix functional theory requires a description of this interaction in terms of natural orbitals (NOs) and their occupation numbers. From an analysis of the configuration-interaction (CI) wave function of the ∑u+3 state of H2 and the exact NO expansion of the two-electron triplet wave function, we demonstrate that the construction of such a functional is straightforward in this case. A quantitative description of the vdW interaction is already obtained with, in addition to the standard part arising from the Hartree-Fock determinant ∫1 σg (r1) 1 σu (r2) ∫, only two additional terms in the two-electron density, one from the first "excited" determinant ∫2 σg (r1) 2 σu (r2) ∫ and one from the state of ∑u+3 symmetry belonging to the (1 π g) 1 (1 π u) 1 configuration. The potential-energy curve of the ∑u+3 state calculated around the vdW minimum with the exact density-matrix functional employing only these eight NOs and NO occupations is in excellent agreement with the full CI one and reproduces well the benchmark potential curve of Kolos and Wolniewicz [J. Chem. Phys. 43, 2429 (1965)]. The corresponding terms in the two-electron density ρ 2 (r1, r2), containing specific products of NOs combined with prefactors that depend on the occupation numbers, can be shown to produce exchange-correlation holes that correspond precisely to the well-known intuitive picture of the dispersion interaction as an instantaneous dipole-induced dipole (higher multipole) effect. Indeed, (induced) higher multipoles account for almost 50% of the total vdW bond energy. These results serve as a basis for both a density-matrix functional theory of van der Waals bonding and for the construction of orbital-dependent functionals in density-functional theory that could be used for this type of bonding. © 2006 American Institute of Physics.
Original languageEnglish
Pages (from-to)054115
JournalJournal of Chemical Physics
Issue number5
Publication statusPublished - 2006


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