Complete active space SCF method

Roos B O, Taylor P R and Siegbahn P E M 1980 A complete active space SCF method (CASSCF) using a density matrix formulated super-CI approach Chem. Phys. 48 157-73... 

Nickel Atom Studied With the Complete Active Space SCF Method and Second-Order Perturbation Theory. 

Roos, B.O. and Taylor, P.R. (1980) A complete active space scf method (Casscf) using a density-matrix formulated super-Ci approach. Chem. Phys., 48, 157-173. 

A conceptually straightforward improvement on the CI approximation is to reoptimize the molecular orbitals for a truncated CI expansion. This approach is called multi-configuration self-consistent field method (MCSCF) and its most prominent variant is the complete active space SCF method (CASSCF) [64]. In the first generation of MCSCF methods [65, 66], the CI coefficients C/ in Eq. 

Roos B O, P R Taylor and E M Siegbahm 1980 A Complete Active Space SCF Method (CASSCF) Using a Density Matrix Formulated Super-CI Approach Chemical Physics 48 157-173. 

How can we extend HF theory to incorporate the effects of the most important natural orbitals, even in cases where the occupation numbers are not close to two or zero Actually, Lowdin gave an answer to this question in his 1955 article, where he derived something he called the extended HF equations. The idea was to use the full Cl wave fimction. Equation (2), but with a reduced number of orbitals, and determine the expansion coefficients and the molecular orbitals variationally. His derivation was formal only and had no impact on the general development at the time. It was not xmtil 20 years later that a similar idea was suggested and developed into a practical computational procedure. The approach is today known as the complete active space SCF method, CASSCF. ... 

Roos, B. O. 1980. The complete active space scf method in a Fock-Matrix-based super-CI formulation./nr. J. Quantum Chem. 17 175-189. 

Siegbahn PEM, Heiberg A, Roos BO, Levy B. Comparison of the super-CI and the Newton-Raphson scheme in the complete active space SCF method. Phys Scr. 1980 21 323. 

The 3D-R1SM-KH solvation method can be combined in a self-consistent field approach with any multireference electronic structure theory. Sato et al. pioneered such a combination of 3D-RISM-KH and ab initio complete active space SCF method. ... 

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