POTENTIAL-ENERGY SURFACE OF THE H-3(-STATE IN THE NEIGHBORHOOD OF THEMINIMUM WITH MICROHARTREE ACCURACY AND VIBRATIONAL FREQUENCIES DERIVED FROM IT() GROUND)

The potential energy surface (PES) of the H-3(+) ground state is compu ted by means of the single and double excitation configuration interac tion with an explicit linear r(12) term in the wave function (CISD-R12 ) developed recently by the present authors, with a nearly saturated b asis set. The points of the PES suggested by Meyer, Botschwina, and Bu rton (MBB) were chosen and the fitting procedure of the same authors w as followed. The present PES has both on an absolute and a relative sc ale (i.e., relative to the minimum) an error of a few microhartrees (m u E(h)) in the relevant region, an accuracy that has never before been achieved in a quantum chemical calculation for a triatomic molecule. From the fit the vibrational term values for the fundamental bands and some overtones of H-3(+), H2D+, HD2+, and D-3(+) were computed by mea ns of the TRIATOM package of Tennyson and Miller. The computed frequen cies are in better agreement with experiment (maximum error similar to 0.5 cm(-1)) than those of all previous ab initio calculations (withou t empirical adjustment). To achieve this accuracy, it is necessary to go beyond the Born-Oppenheimer approximation and to take care of the f inite mass ratio between nuclei and electrons.