Studies of the Interaction between Stable Molecules and Atoms. II. Interaction between Two He Atoms

The repulsive energy of two helium atoms has been calculated using molecular orbital theory. The equations of Roothaan were used to break down the energy into integrals involving atomic orbitals. The necessary integrals are tabulated in the literature for a wide range of parameter which enabled the potential energy curve to be calculated from R=0.6 to 7.0 a0. The Slater (1s) orbital with effective nuclear charge Z=27/16 was used. The potential curve is lower at all distances from those of other investigators in the range where Van der Waals forces give attraction although the curves agree at closer distances. Ionization energies have been calculated over these distances and extrapolated to infinity to give a value which is much better than that calculated with the same orbital by ordinary means. The addition of 2pσ character to the molecular orbitals by constructing hybrid orbitals of the type N {(1s)+λ(2pσ)} and the determination of λ by a SCF calculation according to the scheme of Roothaan, gives an attractive energy of 2.14×10—5 a.u. with λ=1.4×10—3 at R=3.11 A. Experimental values are approximately 3×10—5 a.u. The calculated results are compared with experimental data in the exchange and Van der Waals region of the values of the internuclear parameter.