The interaction-induced dipole of H-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 H ranging from 0.942 a.u. to 2.801 a.u., for 16 different separations R between the centers of mass of H and H between 3.0 a.u. and 10.0 a.u., and for 19 angles θ between the H bond vector r and the vector R from the H 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 H bond axis and of the intermolecular vector, with coefficients D(r, R). For the geometrical configurations that we have studied in this work, the most important coefficients D(r, R) in the series expansion are D(r, R), D(r, R), D(r, R), D(r, R), and D(r, R). We show that the ab initio results for D(r, R) and D(r, R) converge to the classical induction forms at large R. The convergence of D(r, R) to the hexadecapolar induction form is demonstrated for the first time. Close agreement between the long-range ab initio values of D(r = 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, D(r, R) characterizes isotropic overlap and exchange effects, as well as dispersion. The coefficients D(r, R) and D(r, R) represent anisotropic overlap effects. Our results for the D(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 H molecules and H atoms.