Method of Diatomics in Molecules. VI. BeH2

Diatomics-in-molecules theory is applied to linear HBeH using as a basis three canonical valence-bond wavefunctions related to the structures k2s2(a − b), k2(s − a) (z − b), and k2(s − b) ( z− a); k, s, z represent 1s, 2s, and 2pz AO's on Be, and a and b represent 1s AO's on the two hydrogens. Theory indicates that the ground Σ1g+ state is best represented by 24.6% of the first structure and equal weights (37.7%) of the second and third structures. The atomization energy (fixed nuclei) is predicted to be 142 kcal and the optimum RBeH = 2.60 bohrs (cf., about 49 kcal and 2.54 bohrs for ground-state BeH). The Σg+ and Σu+ vibrational frequencies for BeH2 are predicted to be 1841 and 1996 cm−1, respectively (cf., 2059 cm−1 for BeH). Potential-energy surfaces for excited Σ1u+ and Σ1g+ states as well as the ground Σ1g+ state are computed.