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 $k^2{s}^2{\mathrm{(a\hspace{0.17em}-\hspace{0.17em}b),\; k}}^2\mathrm{(s\hspace{0.17em}-\hspace{0.17em}a)\; (z\hspace{0.17em}-\hspace{0.17em}b)}$ , and $k^2\mathrm{(s\hspace{0.17em}-\hspace{0.17em}b)\; (\hspace{0.17em}z-\hspace{0.17em}a)}$ ; $\mathrm{k,\; s,\; z}$ represent $\text{1s, 2s}$ , and $\text{2pz}$ AO's on Be, and $a$ and $b$ represent $\text{1s}$ AO's on the two hydrogens. Theory indicates that the ground ${}^1{\Sigma }_g^{+}$ 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 $R_{\mathrm{BeH}}\text{\hspace{0.17em}=\hspace{0.17em}2.60}\mathrm{bohrs}$ (cf., about 49 kcal and 2.54 bohrs for ground‐state BeH). The ${\Sigma }_g^{+}$ and ${\Sigma }_u^{+}$ 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 ${}^1{\Sigma }_u^{+}$ and ${}^1{\Sigma }_g^{+}$ states as well as the ground ${}^1{\Sigma }_g^{+}$ state are computed.