Application of the Method of Deformed Atoms in Molecules to the Li2 Molecule

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Abstract
The method of deformed atoms in molecules, proposed previously to remedy deficiencies in the method of Moffitt, is employed to calculate energies of the ground state 1Σg+ and the lower excited states 3Σu+, 1Σu+, and 1IIu of the Li2 molecule. As basis for the molecular calculation, modified atomic functions are introduced to take deformation of atoms in molecules into account. These modified functions are constructed from atomic eigenfunctions in a manner which fully preserves correlation between electrons. Exact atomic energies are used to calculate the atomic energy parts of the matrix elements of the molecular calculation, while the energy defects due to deformation of atoms are determined by using approximate functions. The modified atomic functions are made up so that their charge distributions are well described by the use of Slater orbitals with equal orbital exponents for all atomic states. Interatomic interaction operators, overlap integrals, and energy defects due to deformation of atoms are calculated using Slater orbitals. For the 1s)22s)2p) 3P and 1P states of the Li— ion, the exchange correction due to binding a free electron into the 2p orbital is considered. The energy defects of Slater orbitals from Hartree‐Fock orbitals are found to have small effect on the dissociation energy of the ground state. The calculations are performed for three sets of values for the orbital exponents. The dissociation energy obtained for one set of values is 0.96 ev, in good agreement with the observed value of 1.05 ev. The excitation energies obtained of 1.63 ev for 3Σu+—1Σg+, 2.05 ev for 1Σu+—1Σg+ and 2.59 ev for 1IIu—1Σg+ also agree reasonably well with the corresponding observed values of (1.35 ev), 1.76 ev, and 2.54 ev. The calculation is discussed and compared with the orbital approach and Moffitt's method. Conclusions are stated regarding the nature of bonds and orbital exponents in Li2.