C 2υ Potential Energy Surfaces for Seven Low-Lying States of CH2

Ab initio calculations have been carried out at 28 $C_{\text{2υ}}$ geometries for the lowest ${}^1{A}_1{,}^1{A}_2{,}^3{A}_2{,}^1{B}_1{,}^3{B}_1{,}^1{B}_2$ , and ${}^3{B}_2$ states of CH₂. The basis set used was of the contracted Gaussian type with four $s$ and two $p$ functions on carbon and two $s$ functions on hydrogen. In all calculations except ${}^1{A}_1$ the SCF configuration plus all singly and doubly excited configurations were included (holding the $K$ shell frozen), and the iterative natural orbital procedure was used to obtain an optimum set of orbitals. For the ${}^1{A}_1$ state a two configuration SCF calculation was used as the starting point for the configuration interaction calculations. In a preliminary communication we predicted the triplet ground state of CH₂ to be bent, and this prediction has since been justified experimentally by Bernheim et al. and by Wasserman et al. For the ${}^1{A}_1$ state the ab initio geometry is $\text{r\hspace{0.17em}=\hspace{0.17em}1.13 Å, θ\hspace{0.17em}=\hspace{0.17em}104°}$ , compared to experiment, $\text{r\hspace{0.17em}=\hspace{0.17em}1.11 Å, θ\hspace{0.17em}=\hspace{0.17em}102°}$ . For the ${}^1{B}_1$ state the predicted geometry is $\text{r\hspace{0.17em}=\hspace{0.17em}1.09 Å, θ\hspace{0.17em}=\hspace{0.17em}144°}$ , as opposed to experiment, $\text{r\hspace{0.17em}=\hspace{0.17em}1.05 Å, θ\hspace{0.17em}∼\hspace{0.17em}140°}$ . The four other states investigated, ${}^1{A}_2{,}^3{A}_2{,}^1{B}_2$ , and ${}^3{B}_2$ , are all unstable with respect to a carbon atom plus a hydrogen molecule, and it is concluded that there are only four bound non‐Rydberg states of CH₂. The ${}^3{B}_1{–}^1{A}_1$ splitting, unknown experimentally, is predicted to be 7770 cm⁻¹ (0.96 eV). The calculated ${}^1{A}_1{–}^1{B}_1$ splitting is 7860 cm⁻¹ (0.97 eV), compared to the experimental value 7100 cm⁻¹ (0.88 eV). The wave‐functions are discussed and three‐dimensional plots of the potential surfaces are presented.