A reliable new three-dimensional potential energy surface for H2–Kr

An improved three-dimensional potential energy surface for the ${\mathrm{H}}_2–\mathrm{Kr}$ system is determined from a direct fit of new infrared spectroscopic data for ${\mathrm{H}}_2–\mathrm{Kr}$ and ${\mathrm{D}}_2–\mathrm{Kr}$ to a potential energy function form based on the exchange-Coulomb model for the intermolecular interaction energy. These fits require repetitive, highly accurate simulations of the observed spectra, and both the strength of the potential energy anisotropy and the accuracy of the new data make the “secular equation perturbation theory” method used in previous analyses of ${\mathrm{H}}_2$ –(rare gas) spectra inadequate for the present work. To address this problem, an extended version of the “iterative secular equation” method was developed which implements direct Hellmann–Feynman theorem calculation of the partial derivatives of eigenvalues with respect to parameters of the Hamiltonian which are required for the fits.