Energy of electron correlation

The difference between the energy of the system obtained through exact solution to the nonrelativistic Schrodinger equation and the minimal value derived in the HF approximation is customarily regarded as the energy of electron correlation [21]. The point is that the Hamiltonian in the HF method, in particular Eq. (25), includes an averaged interelectronic potential which does not account for correlated motion of electrons in a molecular system. 

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