Complete active self-consistent field calculation

One of the drawbacks of DFT is its inability to treat multiconfigurational problems properly. In these cases, some type of configuration interaction (Cl) method is needed [60], Ideally, one would like to carry out full Cl calculations, but this is generally not possible for practical reasons. Thus, some kind of approximation is needed and, in this respect, the so-called complete active self-consistent field (CASSCF) procedure is often used. 

CASSCF complete active space self-consistent field calculations... 

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... 

CAS-SCF. CAS-SCF (complete active space self-consistent field) calculations select the chemically most significant electrons and orbitals and apply configuration interactions to this set. 

Fig. 1, Eight natural orbitals for the water molecule. They were obtained from a complete active spare self-consistent-field calculation using a basis set of the size (OAs2>p2d/ms2p)...

Roos B O 1987 The complete active space self-consistent field method and its applications in electronic structure calculations Adv. Chem. Phys. 69 399-445... 

It is evident that the approach described so far to derive the electronic structure of lanthanide ions, based on perturbation theory, requires a large number of parameters to be determined. While state-of-the-art ab initio calculation procedures, based on complete active space self consistent field (CASSCF) approach, are reaching an extremely high degree of accuracy [34-37], the CF approach remains widely used, especially in spectroscopic studies. However, for low point symmetry, such as those commonly observed in molecular complexes, the number of CF... 

K. P. Lawley, Ed., chapter 69,399. John Wiley Sons Ltd., Chichester, England, 1987. The Complete Active Space Self-Consistent Field Method and its Applications in Electronic Structure Calculations. 

Density Functional Theory, DFT (B3LYP), CASSCF (Complete Active-State Self-Consistent Field) and MRSD-CI (Multi-Reference Single-Double Correlation Interaction) calculations on the diatomic units AuO, AuO", AuO " and AuO " clearly show that stability of Au-0 bond reduces in this order. This trend is consistent with the molecular orbital diagram of AuO molecule presented in Fig. 10. 

The selection of configuration state functions to be included in MCSCF calculations is not a trivial task. Two approaches which can reduce the complexity of the problem are the complete active space self-consistent-field (CASSCF) [68] and the restricted active space self-consistent-field (RASSCF) [69] approach. Both are implemented in the Dalton program package [57] and are used in this study. Throughout the paper a CASSCF calculation is denoted by i active gactive RASSCF calculation by For the active spaces of HF, H2O, and CH4... 

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

B. O. Roos, The Complete Active Space Self-Consistent Field Method and Its Application in Electronic Structure Calculation, Volume 69 olAdvances in Chemical Physics, Wiley, Chichester, 1987, p. 399. 

B. O. Roos. The complete active space self-consistent field method and its apphca-tion in electronic structure calculations. In K. P. Lawley, editor, Ab Initio Methods in Quantum Chemistry. Part II, volume 69 of Adv. Chem. Phys., pages 399-446. John Wiley, Chichester, 1987. 

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