Interaction Potential between Li+ and H2. II. Region Appropriate for Vibrational Excitation

A self‐consistent‐field (SCF) calculation of the interaction energy between ${\mathrm{Li}}^{+}{(}^1\mathrm{S)}$ and ${\mathrm{H}}_2{\mathrm{(X}}^1{\Sigma }_g^{+})$ has been computed using wavefunctions near the Hartree–Fock limit. An analytical expression for the intermolecular surface has been determined in the form $${\mathrm{V(X,\; \xi ,\; \theta )\hspace{0.17em}=\hspace{0.17em}\upsilon }}_0{\mathrm{(X,\; \xi )\hspace{0.17em}+\hspace{0.17em}\upsilon }}_2{\mathrm{(X,\; \xi )P}}_2(\cos {\mathrm{\theta )\hspace{0.17em}+\hspace{0.17em}\upsilon }}_4{\mathrm{(X,\; \xi )P}}_4(\cos \mathrm{\theta ),}$$ where $X$ is the distance between the Li⁺ and H₂ centers of mass, ${\mathrm{\xi \hspace{0.17em}=\hspace{0.17em}R\hspace{0.17em}-\hspace{0.17em}R}}_e{\mathrm{,\; R}}_e\text{\hspace{0.17em}=\hspace{0.17em}1.4}\mathrm{a.u.}$ , $R$ is the $H_2$ bond distance, $\theta$ is the angle formed by $X$ and $R$ , and is valid for $\text{−\hspace{0.17em}0.4\hspace{0.17em}≤\hspace{0.17em}ξ\hspace{0.17em}≤\hspace{0.17em}0.6}$ and $\text{2.0\hspace{0.17em}≤\hspace{0.17em}X\hspace{0.17em}≤\hspace{0.17em}12.0}\mathrm{a.u.}$ Unlike He–H₂, the Li⁺–H₂ interaction is found to be consistent with the dumbbell model. SCF calculations appear adequate for the calculation of interaction energies for closed‐shell‐ion–closed‐shell‐neutral‐diatomic‐molecule systems even at large $X$ .