$ORMAS group (required by MCSCF if CISTEP=ORMAS) (required for CITYP=ORMAS) This group partitions an active space, defined in $DET or $CIDET, into Occupation Restricted Multiple Active Spaces (ORMAS). All possible determinants satisfying the occupation restrictions (and of course the space symmetry restriction given in $DET/$CIDET) will be generated. This group's usefulness lies in reducing the large number of determinants present in full CI calculations with large active spaces. There are no sensible defaults for these inputs, but if the group is entirely omitted, a full CI calculation will be performed. That is, the defaults are NSPACE=1, MSTART(1)=NCORE+1, MINE(1)=NELS, MAXE(1)=NELS meaning all active orbitals are in one partition. NSPACE = number of orbital groups you wish to partition the active space (NACT in $DET/$CIDET) into. MSTART = an array of NSPACE integers. These specify where each orbital group starts in the full list. You must not overlook the NCORE core orbitals in computing MSTART values. Space I runs from orbital MSTART(I) up to orbital MSTART(I+1)-1, or NACT+NCORE if I is the last space, I=NSPACE. IMPORTANT !!!! Remember to make sure your orbitals have been reordered to suit MSTART, using NORDER in $GUESS. MINE = an array of NSPACE integers. These specify the minimum numbers of electrons that must always occupy the orbital groups. In other words, MINE(I) is the minimum number of electrons that can occupy space I in any of the determinants. MAXE = an array of NSPACE integers. These specify the maximum numbers of electrons that must always occupy the orbital groups. In other words, MAXE(I) is the maximum number of electrons that can occupy space I in any of the determinants. The number of active electrons is NELS in $DET or $CIDET, and the program will check that MINE/MAXE values are consistent with this total number. Input for Coupled Electron Pair Approximations These two keywords provide size-extensivity corrections for singly and doubly excited CI calculations. At present, these approximate corrections may only be applied to a single state. The CISD may follow RHF (single reference, SR) or complete active space (multireference, MR) calculations. Usually, the final ORMAS orbital space will be the entire external space of the SR-CISD or MR-CISD calculations. The excitation level (MAXE) into this space should only be two. The number of electrons being correlated (NELS in $CIDET) is denoted Ne. The value of a parameter G determines the weight of the approximate size- extensivity correction, with the literature containing several possible choices: GVAL here sometimes is written as -(1-G) in the literature. The first order density matrix for the chosen CEPA-style correction will be generated, and used for property calculation. CEPA = NONE ordinary SR-CISD or MR-CISD (default) = CEPA0 Coupled Electron Pair Approximation, whose GVAL= 1.0 = ACPF Averaged Coupled Pair Functional, whose GVAL= (Ne-2)/Ne = AQCC Averaged Quadratic Coupled-Cluster, whose GVAL= (Ne-3)(Ne-2)/[Ne(Ne-1)] = READ user will supply the desired GVAL. GVAL = is given only when CEPA=READ An example of the SR-AQCC size-extensivity correction for a molecule with 10 chemical cores, 8 occupied valence orbitals, and 163 total MOs is $CONTRL SCFTYP=RHF CITYP=ORMAS RUNTYP=ENERGY $CIDET NCORE=10 NACT=153 NELS=16 $ORMAS CEPA=AQCC NSPACE=2 MSPACE(1)=11,19 MINE(1)=14,0 MAXE(1)=16,2 A MR-CEPA case with N active electrons and M electrons in filled valence orbitals should use three spaces, with NELS M+N; MINE M-2,N-2,0; and MAXE M,N+2,2. Review: P.G.Szalay, in Modern Ideas in Coupled-Cluster methods, R.J.Bartlett (ed), World Scientific, Singapore (1997), pp 81-123. * * * * * BLOCK = a flag to request that natural orbital generation for CISTEP=ORMAS MCSCF or CITYP=ORMAS CI runs should prevent mixing between the NSPACE orbital subspaces. This means the NOs only diagonalize the diagonal blocks of the density, and are thus not the genuine NOs. However, these approximate NOs can be used with MOREAD to exactly reproduce the ORMAS energy, which is invariang to rotations within the orbital subspaces. (Default=.FALSE.) QCORR = a flag to request Davidson-style +Q corrections. If this is not sensible for your CI choice, the program will not print this correction, anyway. The default is .TRUE. FDIRCT = a flag to choose storage in memory of some intermediates. This is very large, and slower in the case of many occupied orbitals, but helpful with a smaller number of orbitals. Therefore the default for this is .TRUE. for MCSCF runs, but .FALSE. during CI computations. *** See REFS.DOC for more information on using ORMAS *** ========================================================== ==========================================================

generated on 7/7/2017