Covalent and ionic curve crossings in LiF

Lithium and fluorine form a diatomic molecule that has a large dipole moment in the gas phase it has been measured to be 6.3248 D in the ground vibrational state. The equilibrium intemuclear distance is 1.564 A, and, therefore, the apparent 

If there were no other interaetion the energy of the atoms in the Li+—F state would fall as they approach each other from oo. The energy of this state would cross the ground state at about 12 bohr. In actuality the states interact, and there is an avoided crossing. The energies of the first four states of LiF are shown in Fig. 8.4 as a function of intemuclear distance. In addition, the eleetrie dipole moment is shown. 

The point we wish to bring out here is that the dipole moment curve around 6-7 bohr is very nearly the value expeeted for one eleetronie charge separated by that distanee. As one moves outward, the avoided erossing region is traversed, the state of the moleeule switches over from Li+ —F to Li-F and the dipole falls rapidly. 

The avoided erossing we have diseussed oeeurs between the two etrrves in Fig. 8.4 labeled Ei and 2- Another avoided erossing, farther out than our graph 

The calculations in this illustration were not done with a minimal basis set, since, if such were used, they would not show the correct behavior, even qualitatively. This happens because we must represent both F and F in the same wave function. Clearly one set of AOs cannot represent both states of F. Li does not present such a difficult problem, since, to a first approximation, it has either one orbital or none. The calculations of Fig. 8.4 were done with wave functions of 1886 standard tableaux functions. These support 1020 s mimetry functions. We will discuss the arrangement of bases more fully in Chapter 9. 

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