Because the occupations are proportional to the relative weights of the configurations in which a particular orbital is occupied, one can use natural orbitals to compress the orbital space by dropping the orbitals with low occupation numbers. This utility of natural orbitals to compress correlated wave functions has been exploited in a variety of configuration interaction methods . More recently, hybrid approaches using both localization techniques and natural or pair-natural orbitals have led to the development of very effective computer implementations of modern many-body theories [69-77], enabling, for example, CC calculations on a protein (crambin) consisting of more than 600 atoms and with more than 6000 basis functions .