Electron tunneling, charge transfer, and the intermolecular forces between two H2 molecules

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Abstract
The spherically averaged van der Waals potential for the system H2–H2 has been calculated using a nonorthogonal CI method based on SCF orbitals for the individual molecules. With suitable restrictions on the CI calculation we have isolated various contributions to the van der Waals potential, and have found that charge transfer and dispersion effects are comparable to distances corresponding to the potential minimum. The charge transfer contribution is further discussed in terms of a mechanism involving electron tunneling through the intervening space between the molecules. Our calculation gives a spherically averaged van der Waals well depth of 2.96 meV at 3.49 Å. Experimental values are 3.00 meV at 3.49 Å.