  In quantum chemistry calculations, the correlation energy is defined as the
difference between the Hartree-Fock limit energy and the exact solution of the
nonrelativistic Schrodinger equation. With this definition, the electron
correlation effects are not directly observable. In this report, we show that
the entanglement can be used as an alternative measure of the electron
correlation in quantum chemistry calculations. Entanglement is directly
observable and it is one of the most striking properties of quantum mechanics.
As an example we calculate the entanglement for He atom and H2 molecule with
different basis sets.
