Ab initio calculations of weakly bonded He2 and Be2 molecules by MRCI method with pseudo-natural molecular orbitals

The X1Σ g+ curves of He2 and Be2 have been calculated by extrapolating the BSSE corrected MRCI total energies obtained with large Gaussian basis sets, large reference configuration spaces, and pseudo‐natural molecular orbitals to an infinite basis. The direct calculated He2 nonrelativistic dissociation energies (De) of 11.0031 K is in excellent agreement with the recent theoretical evaluations, whereas the Be2 nonrelativistic De = 822 cm−1 and relativistically corrected De = 818 cm−1 are in good agreement with most known values including the experimental De = 790 ± 30 cm−1. An analysis of the configuration structure of Be2 wave function and calculated vibration spectrum displays unusual type of chemical bond in this molecule and explains a unique form of the Be2 potential curve. Thus, near the equilibrium point the bond can be classified as a conventional covalent bond, whereas at larger distances, it can be classified as van der Waals interaction. The problems of high precision Be2 potential curve calculations are also discussed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011