CIS Keyword
Last Update: 6/26/2001
| Description |
This method keyword requests a calculation on excited states using single-excitation CI (CI-Singles) [87,114].Chapter 9 of Exploring Chemistry with Electronic Structure Methods [304] provides a detailed discussion of this method and its uses.
CI-Singles jobs will also usually 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.
| State Selection Options |
Singlets
Solve only for singlet excited states. Only
effective for closed-shell systems, for which it is the default.
Triplets
Solve only for triplet excited states. Only
effective for closed-shell systems.
50-50
Solve for half triplet and half singlet states.
Only effective for closed-shell systems.
Root=N
Specifies the “state of
interest,” for which the generalized density is to be computed. The
default is the first excited state (N=1).
NStates=M
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).
Add=N
Read converged states off the checkpoint
file and solve for an additional N states.
DENSITY-RELATED OPTIONS
Densities
Compute the one-particle density for each
state.
TransitionDensities
Compute the transition densities
between the ground state and each excited state. The oscillator strengths are
then reported automatically. This is the default.
AllTransitionDensities
Computes the transition
densities between every pair of states.
PROCEDURE- AND ALGORITHM-RELATED OPTIONS
Direct
Forces solution 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 very
large calculations (over 200 basis functions).
MO
Forces solution of the CI-Singles equations using
transformed two-electron integrals. This is the default algorithm in
Gaussian 98. The transformation attempts to honor the MaxDisk
keyword, thus further moderating the disk requirements.
AO
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 and memory.
FC
This indicates “frozen-core,” and it
implies that inner-shells are excluded from the correlation calculation. This
is the default calculation mode. Full specifies the inclusion of all
electrons, and RW and Window allow you to input specific
information about which orbitals to retain in the post-SCF calculation (see the
discussion of the MPn keywords for an example).
Conver=N
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.
Read
Reads initial guesses for the CI-Singles states
off the checkpoint file. Note that unlike the SCF, an initial guess for one
basis set cannot be used for a different one.
Restart
Restarts the CI-Singles iterations off the
checkpoint file.
RWFRestart
Restarts the CI-Singles iterations off the
read-write file. Useful when using non-standard routes to do successive
CI-Singles calculations.
| Debugging Options |
ICDiag
Forces in-core full diagonalization of the
CI-Singles matrix formed in memory from transformed integrals. This is mainly a
debugging option.
MaxDiag=N
Limits the submatrix diagonalized in
the Davidson procedure to dimension N. This is mainly a debugging
option. MaxDavidson is a synonym for this option.
| Availability |
Energies, analytic gradients, and analytic frequencies.
| Related Keywords |
Zindo, TD, Density, MaxDisk, Transformation
| Examples |
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
The iterative process can end successfully in two ways: generation of only vanishingly small expansion vectors, or negligible change in the updated wavefunction.
When 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 excit. 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.