Calculate natural orbitals. This currently only works for closed-shell MP2 and QCISD. The number of printed external orbitals in any given symmetry is nprint) (default 2). nprint=-1 suppressed the printing. The natural orbitals and the density matrix are saved in a dump record record on file ifil. If record.ifil is specified on a DM card (see above), this record is used. If different records are specified on the DM and NATORB cards, an error will result. The record can also be given on the SAVE card. Note that the effective density matrix of non-variational methods like MP2 or QCISD does not strictly behave as a density matrix. For instance, it has non-zero matrix elements between core and valence orbitals, and therefore core orbitals are affected by the natural orbital transformation. Also, occupation numbers of core orbitals can be larger than 2.0. If CORE is given (natcor=1), the core orbitals are frozen by excluding them from the natural orbital transformation.