Ab initio and molecular-dynamics studies on rare gas hydrides:: Potential-energy curves, isotropic hyperfine properties, and matrix cage trapping of atomic hydrogen

Ground-state potential-energy curves and distance dependent isotropic hyperfine coupling (IHC) constants for ground-state H-RG (=Ne, Ar, Kr, Xe) are obtained at CCSD(T) (coupled-cluster single double triple) and MP4(SDQ) (fourth-order Moller-Plesset single double quadruple) levels, respectively, with an augmented basis set aug-Stuttgart (RG)/aug-cc-pVQZ (H). The obtained R-m and epsilon are for NeH: 3.45 Angstrom and -1.36 meV; ArH: 3.65 Angstrom and -3.48 meV; KrH: 3.75 Angstrom and -4.32 meV; XeH: 3.90 Angstrom and -5.22 meV. The computed pair potentials are utilized in classical molecular-dynamics simulations of H-RG lattices. Along the classical trajectory, the many-body perturbation on the H atom hyperfine coupling constant is computed by pair-wise addition of the individual RG-H contributions obtained from the present quantum-chemical calculations. The computed IHC shifts are compared with electron paramagnetic resonance (EPR) spectra obtained in low-temperature matrix isolation experiments. For most cases this theoretical treatment agrees very well with the experiment and confirms the previous site assignments. However, for H-Xe, the theory would suggest stability of both interstitial O-h and substitutional sites, whereas only one site is observed in the experiment. Based on the present calculations this site can be assigned as a nearly undistorted substitutional site. (C) 1999 American Institute of Physics. [S0021-9606(99)31124-7].