Number configuration-interaction method

In the configuration interaction method, the wavefunction is expanded in a basis of Slater determinants. The Hartree-Fock determinant, noted is taken as a reference zeroth-order wavefunction. Slater determinants corresponding to excited configurations are generated by swapping occupied MOs V a with virtual (unoccupied) MOs ipr and can be classified with respect to the number of excited electrons. Singly excited Slater determinants are noted 3, doubly excited Slater determinants triply excited Slater determinants X so on. The configuration interaction wavefunction then reads... 

It is well known that Hartree-Fock (HF) theory not only has been proven to be quite suitable for calculations of ground state (GS) properties of electronic systems, but has also served as a starting point to develop many-parti-cle approaches which deal with electronic correlation, like perturbation theory, configuration interaction methods and so on (see e.g., [1]). Therefore, a large number of sophisticated computational approaches have been developed for the description of the ground states based on the HF approximation. One of the most popular computational tools in quantum chemistry for GS calculations is based on the effectiveness of the HF approximation and the computational advantages of the widely used many-body Mpller-Plesset perturbation theory (MPPT) for correlation effects. We designate this scheme as HF + MPPT, here after denoted HF -f- MP2. ... 

The configuration interaction (CI) method in whieh the LCAO-MO eoeffieients are determined first (and independently) via either a single-eonfiguration SCF ealeulation or an MCSCF ealeulation using a small number of CSFs. The CI eoeffieients are subsequently determined by making the expeetation value < F H F >/< F I F >... 

Cl methods [21] add a certain number of excited Slater determinants, usually selected by the excitation type (e.g. single, double, triple excitations), which were initially not present in the CASSCF wave function, and treat them in a non-perturbative way. Inclusion of additional configurations allows for more degrees of freedom in the total wave function, thus improving its overall description. These methods are extremely costly and therefore, are only applicable to small systems. Among this class of methods, DDCI (difference-dedicated configuration interaction) [22] and CISD (single- and double excitations) [21] are the most popular. 

The ideal calculation would use an infinite basis set and encompass complete incorporation of electron correlation (full configuration interaction). Since this is not feasible in practice, a number of compound methods have been introduced which attempt to approach this limit through additivity and/or extrapolation procedures. Such methods (e.g. G3 [14], CBS-Q [15] and Wl [16]) make it possible to approximate results with a more complete incorporation of electron correlation and a larger basis set than might be accessible from direct calculations. Table 6.1 presents the principal features of a selection of these methods. 

The authors carried out the calculation for a considerable number of aromatic hydrocarbons by this method, in part also including configuration interaction (01). As in the case of anthracene, the possibility of isomeric carbonium ions was taken into account for biphenyl, naphthalene, phenanthrene, pyrene, and perylene. Comparison with the measured spectra permitted a distinction between the isomeric carbonium ions in some cases. The possibility of this differentiation only... 

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