Refined ab initio calculation of the potential energy surface of the He-H2 interaction with special emphasis to the region of the van der Waals minimum

A highly accurate ab initio surface for the interaction potential of the system HeH2 is computed. The method applied is essentially of CI type, but different calculations with different basis sets are performed for (1) the SCF and intrasystem correlations and (2) the intersystem correlation. The former is corrected for basis superposition errors (counterpoise method) and the latter is corrected by inclusion (or simulation) of triply substituted configuration. For either calculation basis saturation tests are performed, a criterion for the interbasis being that it accounts correctly for the van der Waals constants C6,C8 and C10 and their anisotropies on three successive levels of sophistication. The calculations cover the range from R=1.5a0 (0.8 A) to R=infinity for the He--H2 distance, and r=0.9 a0 to 2.0 a0 for the H--H distance and the orientation angles 0 0, 45 0, 90 0