  We present a method to approximate post-Hartree-Fock correlation energies by
using approximate natural orbitals obtained by the random phase approximation
(RPA). We demonstrate the method by applying it to the helium atom, the
hydrogen and fluorine molecule, and to diamond as an example of a periodic
system. For these benchmark systems, we show that RPA natural orbitals converge
the MP2 correlation energy rapidly. Additionally, we calculated full
configuration interaction energies for He and H$_2$, which are in excellent
agreement with the literature and experimental values. We conclude that the
proposed method may serve as a compromise to reach good approximations to
correlation energies at moderate computational cost, and we expect the method
to be especially useful for theoretical studies on surface chemistry by
providing an efficient basis to correlated wave function based methods.
