The interaction-induced dipole of H₂-H: New ab initio results and spherical tensor analysis

We present numerical results for the dipole induced by interactions between a hydrogen molecule and a hydrogen atom, obtained from finite-field calculations in an aug-cc-pV5Z basis at the unrestricted coupled-cluster level including all single and double excitations in the exponential operator applied to a restricted Hartree-Fock reference state, with the triple excitations treated perturbatively, i.e., UCCSD(T) level. The Cartesian components of the dipole have been computed for nine different bond lengths r of H₂ ranging from 0.942 a.u. to 2.801 a.u., for 16 different separations R between the centers of mass of H₂ and H between 3.0 a.u. and 10.0 a.u., and for 19 angles θ between the H₂ bond vector r and the vector R from the H₂ center of mass to the nucleus of the H atom, ranging from 0° to 90° in intervals of 5°. We have expanded the interaction-induced dipole as a series in the spherical harmonics of the orientation angles of the H₂ bond axis and of the intermolecular vector, with coefficients D_λL(r, R). For the geometrical configurations that we have studied in this work, the most important coefficients D_λL(r, R) in the series expansion are D₀₁(r, R), D₂₁(r, R), D₂₃(r, R), D₄₃(r, R), and D₄₅(r, R). We show that the ab initio results for D₂₃(r, R) and D₄₅(r, R) converge to the classical induction forms at large R. The convergence of D₄₅(r, R) to the hexadecapolar induction form is demonstrated for the first time. Close agreement between the long-range ab initio values of D₀₁(r₀ = 1.449 a.u., R) and the known analytical values due to van der Waals dispersion and back induction is also demonstrated for the first time. At shorter range, D₀₁(r, R) characterizes isotropic overlap and exchange effects, as well as dispersion. The coefficients D₂₁(r, R) and D₄₃(r, R) represent anisotropic overlap effects. Our results for the D_λL(r, R) coefficients are useful for calculations of the line shapes for collision-induced absorption and collision-induced emission in the infrared and far-infrared by gas mixtures containing both H₂ molecules and H atoms.