Accurate Potential Energy Surfaces for the study of lithium-hydrogen ionic reactions

Three-dimensional potential energy surfaces (PESs) have been computed, and numerically fitted, for the two lowest electronic states of the LiH system, which are of importance for the astrophysically relevant LiH+ + H -> Li+ + H2 and LiH + H+ -> Li + H exoergic reactions. We extend the recently computed 11 000 multi reference valence bond ab initio energy values [Martinazzo et al., Chem. Phys. 287, 335 (2003)] with 600 multireference configuration interaction calculations with complete active self-consistent field reference functions and a large Li(12s10p4d1f)/H(8s6p3d1f) basis set. We have fitted the full set of energy values with a modified Aguado–Paniagua ansatz that correctly takes into account in this ionic system the important long-range contributions to the potential. Calibration calculations on the three-body potential term and the use of essentially exact results for the two-body contributions allow us to estimate the overall accuracy of the analytic PESs to be within that required for accurate quantum scattering calculations. The above reactions can be treated adiabatically because of the large energy gap separating the two electronic states. The relevant potential energy surfaces have a very different shape. On the one hand, the ground-state PES shows a simple structure, with a downhill route to the products and a shallow well at the C2v geometry which lies 0.286 eV below the Li+ + H2 asymptote. On the other hand, the first excited state is characterized by one deep, dipole-charge well which lies 1.315 eV below the LiH + H+ asymptote, one charge-induced dipole well 0.586 eV below the Li + H asymptote, and a saddle point between them which lies 0.227 eV below the LiH + H+ asymptote. A conical intersection with the second excited state has been found but not yet studied in detail, since we deemed it to be of no direct relevance for the above reactions.