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No restrictions are placed on the occupancy of RAS II orbitals.
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Typically this will correspond to the conventional idea of
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an ``active space'' in multi-reference CI.
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\item[RAS 3 = (integer array)]\mbox{}\\
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\item[RAS3 = (integer array)]\mbox{}\\
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Should be in {\tt psi:()} or {\tt default:()} sections of input.
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As above for {\tt RAS3}, but for the RAS III subspace.
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A maximum of {\tt EX\_LVL} electrons are allowed in RAS III.
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This option allows the user to look for CI vectors of a different irrep
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than the reference. This probably only makes sense for Full CI,
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and it is not supported for unit vector guesses.
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\item[MPN = boolean]\mbox{}\\
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If {\tt TRUE}, compute MPn energies up to nth order, where MAXNVECT = n
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controls the maximum order energy computed. For open-shell systems
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(REF = ROHF, WFN = ZAPTN), ZAPTn energies are computed.
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\end{description}
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For larger computations, additional keywords may be required, as
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described in the \PSIdetci\ man pages.
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\subsubsection{Arbitrary Order Perturbation Theory}
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\PSIthree\ is capable of computing arbitrary order M{\o}ller-Plesset
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perturbation theory (MPn, closed-shell systems) and
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Z-averaged perturbation theory (ZAPTn, open-shell systems) energies,
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invoked with MPN = TRUE. The maximum level of perturbation theory
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computed is controlled by MAXNVECT.
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Higher order energies (2n - 1 and 2n - 2) can be computed at no additional
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computational cost by using WIGNER = TRUE.
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By default, the n{th} order energy is saved, but (2n - 1) or (2n - 2) order
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energies can be saved using SAVE\_MPN2 = 1 or SAVE\_MPN2 = 2, respectively.
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For open-shell systems, arbitary order ZAPTn energies can be computed using
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WFN = ZAPTN and REF = ROHF.
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All other options are the same as