$CIS group                      required when CITYP=CIS
                                 required when CITYP=SFCIS
 
   The CIS method (singly excited CI) is the simplest way
to treat excited states.  By Brillouin's Theorem, a single
determinant reference such as RHF will have zero matrix
elements with singly substituted determinants.  The ground
state reference therefore has no mixing with the excited
states treated with singles only.  Reading the references
given in Section 4 of this manual will show the CIS method
can be thought of as a non-correlated method, rigorously so
for the ground state, and effectively so for the various
excited states.  Some issues making CIS rather less than a
black box method are:
    a) any states characterized by important doubles are
       simply missing from the calculation.
    b) excited states commonly possess Rydberg (diffuse)
       character, so the AO basis used must allow this.
    c) excited states often have different point group
       symmetry than the ground state, so the starting
       geometries for these states must reflect their
       actual symmetry.
    d) excited state surfaces frequently cross, and thus
       root flipping may very well occur.
 
The normal CIS implementation allows the use of only RHF
references, but can pick up both singlet and triplet
excited states. Nuclear gradients are available, as are
properties.  The CIS run automatically includes computation
of the dipole moments of all states, and all pairwise
transition dipoles and oscillator strengths.
 
The spin-flip type of CIS is very similar to spin-flip TD-
DFT (the $TDDFT input contains more information about how
spin-flip runs select the target state's Ms by $CONTRL's
MULT value).  The reference state must be UHF or ROHF, with
MULT in $CONTRL at least 3.  The target states of the CIS
have one lower Ms, after one alpha spin in the reference is
flipped to beta.  Nuclear gradients are possible.
 
Solvent effects are not available for either CIS or SFCIS.
 
It is worthwhile to look at the $TDDFT input, which is a
very similar calculation.  The TD-DFT program offers the
possibility of recovering some of the correlation energy,
permits some solvent models, and can be used for MEX/CONICL
surface intersection searches.
 
The first six keywords are chemically important, while the
remainder are mostly technical.
 
NACORE = n Omits the first n occupied orbitals from the
           calculation (frozen core approximation).
           For CITYP=CIS, the default for n is the number
           of chemical core orbitals.
           For CITYP=SFCIS, the default, which is also the
           only possibility, is 0.
 
NSTATE =   Number of states to be found (excluding the
           reference state).  No default is provided.
 
IROOT  =   State for which properties and/or gradient will
           be calculated.  Only one state can be chosen.
           The reference state is referred to as 0, and in
           the case of CITYP=SFCIS, might have a higher
           energy than some of the NSTATE target states.
 
CISPRP =   Flag to request the determination of CIS level
           properties, using the relaxed density.  Relevant
           to RUNTYP=ENERGY jobs, although the default is
           .FALSE. because additional CPHF calculation will
           be required.  Properties are an automatic by-
           product of runs involving the CIS or SFCIS
           nuclear gradient.
 
HAMTYP =   Type of CI Hamiltonian to use, if CITYP=CIS.
       =   SAPS spin-adapted antisymmetrized product of
                the desired MULT will be used (default)
       =   DETS determinant based, so both singlets and
                triplets will be obtained.
 
MULT   =   Multiplicity (1 or 3) of the singly excited
           SAPS (the reference can only be singlet RHF).
           Only relevant for SAPS-based CITYP=CIS run,
           as SFCIS controls the Ms for target states by
           the value of MULT in $CONTRL.
 
                    - - - - - - - - - - - -
 
DIAGZN =   Hamiltonian diagonalization method.
       =   DAVID use Davidson diagonalization.  (default)
       =   FULL  construct the full matrix in memory and
                 diagonalize, thus determining all states
                 (not recommended except for small cases).
 
DGAPRX =   Flag to control whether approximate diagonal
           elements of the CIS Hamiltonian (based only on
           the orbital energies) are used in the Davidson
           algorithm.  Note, this only affects the rate of
           convergence, not the resulting final energies.
           If set .FALSE., the exact diagonal elements are
           determined and used.  Default=.TRUE.
 
NGSVEC =   Dimension of the Hamiltonian submatrix that is
           diagonalized to form the initial CI vectors.
           The default is the greater of NSTATE*2 and 10.
 
MXVEC  =   Maximum number of expansion basis vectors in the
           iterative subspace during Davidson iterations,
           before the expansion basis is truncated.  The
           default is the larger of 8*NSTATE and NGSVEC.
 
NDAVIT =   Maximum number of Davidson iterations.
           Default=50.
 
DAVCVG =   Convergence criterion for Davidson eigenvectors.
           Eigenvector accuracy is proportional to DAVCVG,
           while the energy accuracy is proportional to its
           square.  The default is 1.0E-05.
 
CHFSLV =   Chooses type of CPHF solver to use.
       =   CONJG selects an ordinary preconditioned
                 conjugate gradient solver.  (default)
       =   DIIS  selects a diis-like iterative solver.
 
RDCISV =   Flag to read CIS vectors from a $CISVEC input
           group in the input file.  Default is .FALSE.
 
MNMEDG =   Flag to force the use of the minimal amount of
           memory in construction of the CIS Hamiltonian
           diagonal elements.  This is only relevant when
           DGAPRX=.FALSE., and is meant for debug purposes.
           The default is .FALSE.
 
MNMEOP =   Flag to force the use of the minimal amount of
           memory during the Davidson iterations. This is
           for debug purposes. The default is .FALSE.
 
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