Configuration interaction—singles-only

The simplest and most widely-employed method is the so-called configuration interaction singles or CIS method. This involves singleelectron promotions only (from occupied molecular orbitals in the reference wavefunction to unoccupied molecular orbitals). Because there are relatively few of these, CIS is in fact practical for molecules of moderate complexity. As noted previously, single-electron promotions do not lead to improvement in either the ground-state wavefunction or energy over the corresponding Hartree-Fock... 

CIS Model. Configuration Interactions Singles. A limited Configuration Interaction scheme in which only single excitations from occupied to unoccupied molecular orbitals are considered. This is perhaps the simplest method available to the description of Excited States of molecules. 

CIS Configuration interaction including only single excitations from the Hartree-Fock MO determinant. A simple method for the calculations of excited states. 

Table 2.7 contains five columns of numbers for excitation energies in Be. The first two are from Ref. [106]. The acronyms SC-SF-CIS and FCI stand for "spin-complete, spin-flip, configuration-interaction singles" and "full configuration interaction," respectively. In this case, the application of SPSA employs the differences of only the Fermi-sea energies given in the previous subsection (15a, 16c, and 19). Also, the Fermi-sea wavefunction for the... 

In order to calculate nonadiabatic couplings in the framework of the TDDFT method a representation of the wavefunction based on Kohn-Sham (KS) orbitals is required. Since in the linear response TDDFT method the time-dependent electron density contains only contributions of single excitations from the manifold of occupied to virtual KS orbitals, a natural ansatz for the excited state electronic wavefunction is the configuration interaction singles (ClS)-Uke expansion ... 

A configuration interaction calculation is available only for single points when the reference ground state is obtained from an RHF calculation. 

The UHF option allows only the lowest state of a given multiplicity to be requested. Thus, for example, you could explore the lowest Triplet excited state of benzene with the UHF option, but could not ask for calculations on an excited singlet state. This is because the UHF option in HyperChem does not allow arbitrary orbital occupations (possibly leading to an excited single determinant of different spatial symmetry than the lowest determinant of the same multiplicity), nor does it perform a Configuration Interaction (Cl) calculation that allows a multitude of states to be described. 

Practical configuration interaction methods augment the Hartree-Fock by adding only a limited set of substitutions, truncating the Cl expansion at some level of substitution. For example, the CIS method adds single excitations to the Hartree-Fock determinant, CID adds double excitations, CISD adds singles and doubles, CISDT adds singles, doubles, and triples, and so on. 

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