Diatomic Hydrides of First-row Atoms, AH

3 Diatomic Hydrides of First-row Atoms, AH A. LiH and LiH .—LiH has been a favourite molecule of theoreticians, and many calculations have been reported in recent years. Early work has been listed in Richards bibliography,3 and Browne and Matsen7 have discussed more recent work. Calculations of molecular properties for a variety of small molecules, including LiH and various other hydrides, have been reviewed by Lipscomb.103 

The bulk of previous work has concentrated on the X1S+ ground state, but there is now an increasing number of studies on the excited states, particularly with wave-functions beyond Hartree-Fock, and therefore including electron correlation. For comparison with this work, the near-HF calculation of Cade and Huo104 remains a useful comparison point. 

Docken and Hinze108 have presented a very detailed study of the potential-energy curves for five valence excited states of LiH by the MCSCF method. In this type of calculation, the wavefunction, expressed in the form (5), is variationally optimized 

Although the emphasis in ab initio calculations in recent years has been on calculations using the MO approach, alternative methods have not been neglected, particularly Valence Bond (VB) methods of various types. Particularly promising has been the work of Goddard and co-workers with the so-called Generalized Valence Bond (GVB) method.13 

The GVB method avoids a major deficiency of the HF method, namely the incorrect dissociation behaviour of the wavefunction, by taking the wavefunction to be of the form (6),108 where GJ is a group operator that simultaneously takes care of 

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