CIS method keyword requests a calculation on excited states using single-excitation
CI (CI-Singles) . Chapter 9 of Exploring
Chemistry with Electronic Structure Methods 
provides a detailed discussion of this method and its uses.
keyword and option is used to request the related CIS(D) method [426,427].
You can also follow a CIS job with a CIS(D) job to compute the excitation
energies for additional states (see the examples).
CIS jobs can include
the Density keyword; without options, this
keyword causes the population analysis to use the current (CIS) density rather
than its default of the Hartree-Fock density. Note that Density cannot
be used with CIS(D).
STATE SELECTION OPTIONS
Solve only for singlet excited states. This option only affects calculations on
closed-shell systems, for which it is the default.
Solve only for triplet excited states. This option only affects calculations on
Solve for half triplet and half singlet
states. This option only affects calculations on closed-shell systems.
Specifies the "state of interest" for which the generalized density is to be computed.
The default is the first excited state (N=1).
Solve for M states (the default is 3). If 50-50 is requested, NStates
gives the number of each type of state for which to solve (i.e., the default is
3 singlets and 3 triplets).
states off the checkpoint file and solve for an additional N states. This
option implies Read as well. NStates cannot be used with this option.
Computes the transition
densities between every pair of states.
PROCEDURE- AND ALGORITHM-RELATED
All frozen core options are
available with CIS and CIS(D).
of the CI-Singles equation using AO integrals which are recomputed as needed.
CIS=Direct should be used only when the approximately 4O2N2
words of disk required for the default (MO) algorithm are not available,
or for larger calculations (over 200 basis functions).
solution of the CI-Singles equations using transformed two-electron integrals.
This is the default algorithm in Gaussian 03. The transformation
attempts to honor the MaxDisk keyword, thus further moderating the disk
Forces solution of the CI-Singles equations
using the AO integrals, avoiding an integral transformation. The AO basis is seldom
an optimal choice, except for small molecules on systems having very limited disk
Sets the convergence calculations
to 10-N on the energy and 10-(N+2) on the wavefunction.
The default is N=4 for single points and N=6 for gradients.
Reads initial guesses for the CI-Singles states off the checkpoint file. Note
that, unlike for SCF, an initial guess for one basis set cannot be used for a
Restarts the CI-Singles iterations off
the checkpoint file. Also implies SCF=Restart.
Restarts the CI-Singles iterations off the read-write file. Useful when using
non-standard routes to do successive CI-Singles calculations.
Whether to perform equilibrium or non-equilibrium PCM solvation. NonEqSolv
is the default.
Forces the use of canonical single
excitations for the guess. IVOGuess, which uses improved virtual orbitals,
is the default.
full diagonalization of the CI-Singles matrix formed in memory from transformed
integrals. This is mainly a debugging option.
Limits the submatrix diagonalized in the Davidson procedure to dimension N.
This is mainly a debugging option. MaxDavidson is a synonym for this option.
analytic gradients, and analytic frequencies for CIS, and energies for
Output. There are no special features or pitfalls with CI-Singles input. Output
from a single point CI-Singles calculation resembles that of a ground-state CI
or QCI run. An SCF is followed by the integral transformation and evaluation of
the ground-state MP2 energy. Information about the iterative solution of the CI
problem comes next; note that at the first iteration, additional initial guesses
are made, to ensure that the requested number of excited states are found regardless
of symmetry. After the first iteration, one new vector is added to the solution
for each state on each iteration.
The change in excitation energy and wavefunction
for each state is printed for each iteration (in the #P output):
Iteration 3 Dimension 27
Root 1 not converged, maximum delta is 0.002428737687607
Root 2 not converged, maximum delta is 0.013107675296678
Root 3 not converged, maximum delta is 0.030654755631835
Excitation Energies [eV] at current iteration:
Root 1 : 3.700631883679401 Change is -0.001084398684008
Root 2 : 7.841115226789293 Change is -0.011232152003400
Root 3 : 8.769540624626156 Change is -0.047396173133051
iterative process can end successfully in two ways: generation of only vanishingly
small expansion vectors, or negligible change in the updated wavefunction.
the CI has converged, the results are displayed, beginning with this banner:
Excited States From <AA,BB:AA,BB> singles matrix:
The transition dipole moments between the ground and each excited state
are then tabulated. Next, the results on each state are summarized, including
the spin and spatial symmetry, the excitation energy, the oscillator strength,
and the largest coefficients in the CI expansion (use IOp(9/40=N)
to request more coefficients: all that are greater than 10-N):
Excitation energies and oscillator strengths:
symmetry excitation energy oscillator strength
Excited State 1: Singlet-A" 3.7006 eV 335.03 nm f=0.0008
8 -> 9 0.69112 CI expansion coefficients for each excitation.
Excitation is from orbital 8 to orbital 9
This state for opt. and/or second-order corr. => This is the "state of interest.
Total Energy, E(Cis) = - 113.696894498 CIS energy is repeated here for convenience.
Normalization. For closed shell calculations, the sum of the squares
of the expansion coefficients is normalized to total 1/2 (as the beta
coefficients are not shown). For open shell calculations, the normalization
sum is 1.
Finding Additional States.
The following route will read the CIS results from the checkpoint file
and solve for 6 additional states beyond the second state:
The same procedure will work using CIS(D) in the follow-up job.