Configuration interaction selection

The effects of electron correlation are investigted through the CIPSI (Configuration Interaction with Perturbatively Selected Configurations) calculations (4) of the molecular states. 

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. 

I. Selection of parameters and the basis of configuration interaction in closed shell and restricted open shell semiempirical methods. J. Phys. Chem. 77, 107 (1973). 

ELECTRONIC EXCITATIONS OF NAPHTHALENE SELECTION RULES AND CONFIGURATION INTERACTION... 

Complete Cl, or full Cl, is configuration interaction with a configuration list which includes all possible configurations of proper spin and space symmetry in the chosen orbital space. As has been mentioned previously, the number of configurations in complete Cl will depend in an n-factorial way on the number of electrons and the number of orbitals and it will therefore quickly become too large to be handled. This method is therefore not very well suited as a standard model to solve quantum chemical problems. There are, however, two situations where an efficient complete Cl method is useful to have. The first of these is in connection with the CASSCF method which has been described in another chapter. The other is in connection with bench mark tests. Since any other Cl method selects configurations after some principle, a comparison to complete Cl is the way to check these principles out. We will therefore in this section briefly outline the main steps in the complete Cl method as it is carried out today. 

P. Bogdanovich, R. Karpuskiene, A. Momkauskaite, A program of generation and selection of configurations for the configuration interaction method in atomic calculations SELECTCONF, Comput. Phys. Commun., 172, 133-143 (2005). 

An implementation of the configuration-selecting multireference configuration-interaction method on massively parallel architectures 95... 

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