Cis procedures

Beyond Two-Body Interaction Fragment-Localized Kohn-Sham Orbitals via a Singles-CI Procedure... 

Reinhardt P, Piquemal J-P, Savin A (2008) Fragment-localized Kohn-Sham orbitals via a Singles-CI procedure and application to local properties and intermolecular energy decomposition analysis. J Chem Theory Comput 4 2020... 

Figure 14.5 The CIS procedure diagonalizes the CI matrix formed only from the HF reference and all singly excited configurations. The diagonalization provides energy eigenvalues and associated eigenvectors that may be used to characterize individual states as linear combinations of single excitations...

In fact, the distinction between two-step and direct dynamics is rather fuzzy. The basic issue is what kind and amount of preliminary work is needed before starting a dynamical calculation. Direct ab initio dynamics [90,97-101] requires a minimum of preparation some tests to choose basis sets and other options may suffice. For large systems, however, fully ab initio calculations are impractical, and one has to resort to QM/MM or PCM approaches but then, a host of empirical parameters are introduced, which may need some readjustement to avoid artefacts and to improve the accuracy before starting the dynamical calculations. The same holds for the semiempirical methods in order to represent at best the excited states, one has to re-parameterize the hamiltonian. In particular, our FOMO-SCF-CI method [56-58] differs considerably from the normal SCF or SCF+CIS procedures, so that the standard parameters need to be modified. However, the parameter sets are fairly transferable, and their optimization can be limited to the atoms belonging to the chromophore. In the two-step strategies one fits the ab... 

As already pointed out, the super-CI procedure can be regarded as an approximate version of the augmented Hessian approach. This is of course also true for the method described above, the only difference being that the approximations made are more severe. 

From a practical point of view, errors in transition energies are of the order of 0.2 eV if a standard AO and MRD-CI treatment is employed in the calculation, and similar errors can occur across potential surfaces. Standard treatments are thereby those which employ a double-zeta AO basis plus some polarization/correlation functions (normally one d function) and the necessary spectroscopic orbitals, and employ reference sets whose total contribution to the Cl expansion is at least 90%. In order to insure higher accuracy, the AO set has to be increased in accordance with the discussion in Section II.B and the error limits in the MRD-CI procedure have to be evaluated in the manner outlined above. 

The idea to combine a method only polynomial (Eq.6 with 5 0 and c = 0) with the SCF-CI procedure (Eq.5 with initially developed for the calculation of magnetic observables (9) and later for the electric ones (10). Thus, the first-order perturbed wavefunction is given by ... 

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