We discuss the use of natural orbitals as single-particle basis states for the Equation of Motion Phonon Method (EMPM). They are obtained by computing a ground-state one-body density matrix in second-order many-body perturbation theory.
We focus our attention on energy and proton point radius of O-16 and show that, with respect to Hartree-Fock, the new basis improves drastically the convergence of the two-phonon correlation energy.