Stable leading subspace

Instead of working in a stable subspace of (i.e. in a basis of lOp determinants), one may work in a basis of a rather limited number of configurations, those which play a major role in the adiabatic eigenfunctions of interest, and build an effective electronic Hamiltonian in this model space. For instance, for the curve crossing between the ionic and neutral configuration in NaCl, one may define as a model space the two leading configurations... 

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