Parameterized configuration interaction method

The Parameterized Configuration Interaction (PCI-X) method simply takes the correlation energy and scales it by a constant factor X (typical value 1.2), i.e. it is assumed that the given combination of method and basis set recovers a constant fraction of the correlation energy. 

Since the establishment of necessary basis for accurate computation, simpler approaches with appropriate parameterization have been developed. In particular, parameterization of the PPP-SCF-CI (Pariser, Parr, Pople-self-consistent-field-configuration interaction) method has enabled /8-values (of sufficient accuracy to provide a basis for molecular design) to be obtained with much less powerful computers. Routines are now available for microcomputers that run in reasonable time with adequate accuracy [20]. 

A more widely used approach for organic molecules is based on second-order perturbation theory. Here the dipolar contribution to the field induced charge displacement is calculated by inclusion of the optical field as a perturbation to the Hamiltonian. Since the time dependence of the field is included here, dispersion effects can be accounted for. In this approach the effect of the external field is to mix excited state character into the ground state leading to charge displacement and polarization. The accuracy of this method depends on the parameterization of the Hamiltonian in the semi-empirical case, the extent to which contributions from various excited states are incorporated into the calculation, and the accuracy with which those excited states are described. This in turn depends on the nature of the basis set and the extent to which configuration interaction is employed. This method is generally referred to as the sum over states (SOS) method. 

Configuration Interaction Semiempirical Calculations MNDO MNDO/d Parameterization of Semiempirical MO Methods PM3 Semiempirical Vibrational Frequencies (Including Scaling). 

The parameterization of the exo-anomeric terms was based on the difference between the energy surfaces for 2-methoxytetrahydropyran (2) and 2-ethyltetrahydropyran (3) (see Figure 1), as calculated with the semi-empirical PCILO (perturbation of the configuration interaction matrix from localized bond orbitals) method (equations 7 and 8). ... 

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