HIGH-QUALITY THEORETICAL POTENTIAL-ENERGY SURFACE FOR BE-2 BY USING THE MULTIREFERENCE AVERAGED QUADRATIC COUPLED-CLUSTER (MR-AQCC) METHOD AND LARGE BASIS-SETS

Both electron correlation and basis set effects are important in the d escription of the bond in Be,. We apply a new version of the multirefe rence averaged quadratic coupled-cluster method which is an approximat ely size extensive modification of the multireference configuration in teraction method and was shown to yield potential energy surfaces para llel to full-CI ones. We show this in the cast of Be-2 as well. Since the method is considerably cheaper than full-Ct, we were able to accou nt for core correlation and calculate the whole potential energy surfa ce. Equilibrium geometry, vibrational frequencies and dissociation ene rgy are obtained. For the former two quantities the experimental data are confirmed by the calculations, while the results suggest that the dissociation energy might be considerably higher than the one obtained from the lowest vibrational frequencies. Special attention is paid to the basis set superposition error. Our best calculation of the dissoc iation energy D-e is 864 cm(-1) which is considered to be a lower boun d.