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$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 $CISVEC input
group in the input file. Default is .FALSE.
MNMEDG = Flag to force the use of the minimal amount of
memory in construction of the CIS Hamiltonian
diagonal elements. This is only relevant when
DGAPRX=.FALSE., and is meant for debug purposes.
The default is .FALSE.
MNMEOP = Flag to force the use of the minimal amount of
memory during the Davidson iterations. This is
for debug purposes. The default is .FALSE.
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$CISVEC group required if RDCISV in $CIS is chosen
This is formatted data generated by a previous CIS run, to
be read back in as starting vectors. Sometimes molecular
orbital phase changes make these CI vectors problematic.
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generated on 7/7/2017