# The interaction-induced dipole of H-2-H: New ab initio results and spherical tensor analysis

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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-2 ranging from 0.942 a.u. to 2.801 a.u., for 16 different separations R between the centers of mass of H-2 and H between 3.0 a.u. and 10.0 a.u., and for 19 angles theta between the H2 bond vector r and the vector R from the H-2 center of mass to the nucleus of the H atom, ranging from 0 degrees to 90 degrees in intervals of 5 degrees. We have expanded the interaction-induced dipole as a series in the spherical harmonics of the orientation angles of the H-2 bond axis and of the intermolecular vector, with coefficients D-lambda L(r, R). For the geometrical configurations that we have studied in this work, the most important coefficients D?L(r, R) in the series expansion are D-01(r, R), D-21(r, R), D-23(r, R), D-43(r, R), and D-45(r, R). We show that the ab initio results for D-23(r, R) and D-45(r, R) converge to the classical induction forms at large R. The convergence of D-45(r, R) to the hexadecapolar induction form is demonstrated for the first time. Close agreement between the long-range ab initio values of D-01(r(0) = 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-01(r, R) characterizes isotropic overlap and exchange effects, as well as dispersion. The coefficients D-21(r, R) and D-43(r, R) represent anisotropic overlap effects. Our results for the D-lambda L(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-2 molecules and H atoms. Published under license by AIP Publishing.