Helium-Atom–Hydrogen-Molecule Potential Surface Employing the LCAO–MO–SCF and CI Methods

Wavefunctions and interaction energies are calculated for the helium‐atom–hydrogen‐molecule system at a wide range of internuclear separations. Letting $X$ represent the distance measured along a line drawn from the helium nucleus to the midpoint of the hydrogen molecule, $R$ the H₂ bond distance, and $\theta$ the angle between $X$ and $R$ , results are reported for $\text{2.8\hspace{0.17em}≤\hspace{0.17em}X\hspace{0.17em}≤\hspace{0.17em}7.0}\mathrm{a.u}\text{., 1.0\hspace{0.17em}≤\hspace{0.17em}R\hspace{0.17em}≤\hspace{0.17em}1.8}\mathrm{a.u}\text{. (Re\hspace{0.17em}=\hspace{0.17em}1.4}\mathrm{a.u}\mathrm{.)}$ , and $\text{0°\hspace{0.17em}≤\hspace{0.17em}θ\hspace{0.17em}≤\hspace{0.17em}90°}$ . The orbital basis set consisted of Slater $s$ and $p$ orbitals, and all integrals are accurately evaluated. The wavefunctions are constructed by each of the two major approaches to molecular bonding; the self‐consistent‐field molecular‐orbital method of Roothaan and the configuration‐interaction method. The value of the two methods in atom–diatomic‐molecule interactions is compared extensively. For each individual method, the effect of the size of the basis set on the resultant interaction energy is examined. The values for the He–H₂ interaction energies obtained by the self‐consistent‐field and configuration‐interaction method are very similar. The self‐consistent‐field method appears adequate for the calculation of interaction energies for closed‐shell noninteracting systems except at large center‐of‐mass separations. The interaction energies are accurately fit to an analytic expression for the intermolecular potential. Configuration‐interaction calculations performed at large center‐of‐mass separations fail to locate the He–H₂ van der Waals minimum.