Multiconfigurational self-consistent field MCSCF technique

In this substection we will shortly discuss the computational methods used for calculation of the spin-spin coupling constants. Two main approaches available are ab initio theory from Hartree-Fock (or self-consistent field SCF) technique to its correlated extensions, and density function theory (DFT), where the electron density, instead of the wave function, is the fundamental quantity. The discussion here is limited to the methods actually used for calculation of the intermolecular spin-spin coupling constants, i. e. multiconfigurational self consistent field (MCSCF) theory, coupled cluster (CC) theory and density functional theory (DFT). For example, the second order polarization propagator method (SOPPA) approach is not... 

In Chapter 12, we study the related multiconfigurational self-consistent field (MCSCF) method, in which a simultaneous optimization of orbitals and Cl coefficients is attempted. Although the MCSCF method is incapable of providing accurate energies and wave functions, it is a flexible model, well suited to the study of chemical reactions and excited states. This chapter concentrates on techniques of optimization, a difficult problem in MCSCF theory because of the simultaneous optimization of orbitals and Cl coefficients. 

In the Cl methods mentioned so far, only the mixing coefficients of the excited configurations are optimized in the variational calculations. If we optimize both the coefficients of the configurations and those of the basis functions, the method is called MCSCF, which stands for multiconfiguration self-consistent field calculation. One popular MCSCF technique is the complete active-space... 

We have already presented [17,18] the SCF-Ml (Self Consistent Field for Molecular Interactions) method, based on the idea that BSSE can be avoided a priori provided the MOs of each fragment are expanded only using basis functions located on each subsystem. In the present work we propose a multiconfiguration extension (MCSCF-MI) of the same technique, particularly suited to deal with systems for which proton transfer processes must be considered. 

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