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24 THE CLOSED SHELL CCSD PROGRAM 186<br />

24.8 Natural orbitals<br />

NATORB,[RECORD=]record.ifil,[PRINT=nprint],[CORE[=natcor]];<br />

Calculate natural orbitals. This currently only works for closed-shell MP2 and QCISD. The<br />

number of printed external orbitals in any given symmetry is nprint) (default 2). nprint=-1<br />

suppressed the printing. The natural orbitals and the density matrix are saved in a dump record<br />

record on file ifil. If record.ifil is specified on a DM card (see above), this record is used. If<br />

different records are specified on the DM and NATORB cards, an error will result. The record<br />

can also be given on the SAVE card. Note that the effective density matrix of non-variational<br />

methods like MP2 or QCISD does not strictly behave as a density matrix. For instance, it has<br />

non-zero matrix elements between core and valence orbitals, and therefore core orbitals are<br />

affected by the natural orbital transformation. Also, occupation numbers of core orbitals can<br />

be larger than 2.0. If CORE is given (natcor=1), the core orbitals are frozen by excluding them<br />

from the natural orbital transformation.

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