Configuration Interaction Study of the Lithium Hydride Molecule

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Abstract
A set of orbitals in elliptical coordinates has been used as basis in a self‐consistent field configuration interaction (SCF–CI) study of the electronic structure of the ground state (1Σ+) of the lithium hydride molecule. The importance of various configurations is studied, and it is found that those configurations are most important which have either the inner‐shell molecular orbital or the valence‐shell molecular orbital doubly occupied. The results of each configuration interaction calculation have been analyzed into contributions to the inner‐shell and outer‐shell correlation energies, and in this way the importance of various orbitals is studied and related to correlation effects. It appears that LiH could be well described by a product of electron‐pair wave functions. A study of the numerical stability of the calculations shows that considerable care must be exercised in the choice of a basis in order for the results to be meaningful. Instability of a calculation is due to near linear dependency of the orbital basis. The best total energy obtained, using 53 configurations formed from a basis of 7σ and 3π orbitals, is —8.04128H at an internuclear distance of 2.99B. This should be compared with the experimental energy of —8.0703H. The dipole moment was calculated to be 5.96D, and the ratio of dipole moment to dipole derivative is μe/(δμ/δr)ere=1.53 (experimental values are 5.88D and 1.8±0.3, respectively).