Entanglement is a concept commonly used with reference to the existence of certain correlations in quantum systems that have no classical interpretation and it holds a fundamental role in quantum information theory. Starting from the Collin’s conjecture, that is, the correlation energy in molecular systems is proportional to their entropy, we have interpreted the degree of entanglement as an evaluation of correlation energy. Entanglement is in fact a physical observable directly measured by the von Neumann entropy of the system. This concept is used in order to give a physical meaning to the electron correlation energy, which is defined as the difference between the total energy of a given molecular system, with respect to the one obtained with the Hartree–Fock approximation method. We have made a measurement of electron entanglement and compared with electron correlation energy in two different examples of …