Bare interaction energy, renormalization

Actually, t is a bare value in the Hamiltonian. When a one-electron picture is taken, 4t is the bandwidth. However, electron-electron interaction can largely renormalize t. This fact allows us to apply different t for different U,V) pairs in estimating both exciton energy and its binding energy when comparing with experiment. 

The renormalization of the bare i2-subsystem Hamiltonian leads to addition of some interaction terms to the sum of energies of free subsystems. The PESs for the whole molecule are obtained by formula ... 

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