Configuration interaction models

Kallay, M., Gauss, J., Szalay, P.G. Analytic first derivatives for general coupled-cluster and configuration interaction models. J. Chem. Phys. 2003, 119, 2991-3004. 

This chapter reviews models based on quantum mechanics starting from the Schrodinger equation. Hartree-Fock models are addressed first, followed by models which account for electron correlation, with focus on density functional models, configuration interaction models and Moller-Plesset models. All-electron basis sets and pseudopotentials for use with Hartree-Fock and correlated models are described. Semi-empirical models are introduced next, followed by a discussion of models for solvation. 

Conceptually, the most straightforward approach is the so-called full configuration interaction model. Here, the wavefunction is written as a sum, the leading term of which, Fo, is the Hartree-Fock wavefunction, and remaining terms, Fs, are wavefunctions derived from the Hartree-Fock wavefunction by electron promotions. 

Size consistency is a more important attribute than variational, and because of this, Moller-Plesset models are generally preferred over configuration interaction models. 

Correlated Models. Models which take implicit or explicit account of the Correlation of electron motions. Moller-Plesset Models, Configuration Interaction Models and Density Functional Models... 

Size Consistent. Methods for which the total error in the calculated energy is more or less proportional to the (molecular) size. Hartree-Fock and Moller-Plesset models are size consistent, while Density Functional Models, (limited) Configuration Interaction Models and Semi-Empirical Models are not size consistent. 

Variational. Methods for which the calculated energy represents an upper bound to the exact (experimental) energy. Hartree-Fock and Configuration Interaction Models are variational while Moller-Plesset Models, Density Functional Models and Semi-Empirical Models are not variational. 

Following Refs. [61, 62], a two-band (valence and conduction band) configuration interaction model is introduced, using a basis of monoexcited configurations on the polymer chain. These correspond to electron-hole states n) = nen h) = ne)c <8> n h)v localized at sites n and n of the chain. Here,... 

This configuration interaction model will have an important bearing on the intensities of the bands at 16 and 20 kK. In derivatives which lie to the left of Fig. 10, the 16 kK band will be mostly charge transfer, but as we move towards the right-hand side of the diagram, this band will acquire an increasing proportion of n - n character. Since we suppose... 

R. J. Bartlett and I. Shavitt, Determination of the Size-Consistency Error in the Single and Double Excitation Configuration Interaction Model, Int. 

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