NATURAL ORBITAL FUNCTIONAL FOR THE MANY-ELECTRON PROBLEM

The exchange-correlation energy in Kohn-Sham density functional theory is expressed as a functional of the electronic density and the Kohn-S ham orbitals. An alternative to Kohn-Sham theory is to express the ene rgy as a functional of the reduced first-order density matrix or equiv alently the natural orbitals. We present an approximate, simple, and p arameter-free functional of the natural orbitals, based solely on scal ing arguments and the near satisfaction of a sum rule. Our tests on at oms show that it yields on average more accurate energies and charge d ensities than the Hartree-Fock method, the local density approximation , and the generalized gradient approximations.