Unitary group approach applications

At the ab-initio level, the most obvious possibility is offered by CAS SCF or CAS FCI (i.e., Cl within the CAS or, equivalently, CAS SCF without the orbital reoptimization based on RHF orbitals, cf. [33, 34]) wave functions based on the smallest possible active-space that warrants the correct description of the dissociation channel at hand. This option was also suggested by Stolarczyk [29], although we are not aware of any concrete implementation. Our testing proved to be very encouraging [33, 34], particularly for open shell systems, in which case we employed the spin-adapted CCSD based on the unitary group approach (UGA) [16, 36]. Even in the case of triple bond breaking, the applicability of the CCSD approximation can be significantly extended, as will be shown in Sect. 4. Most recently, we have explored the MR CISD wave function as an external source, as described in the next section. 

It should be pointed out, however, that the size of the Cl expansion used in a CASSCF calculation is almost always much larger than those normally used in earlier applications of the multiconfigurational SCF (MCSCF) method . It was only when the graphical unitary group approach (GUGA) for full Cl calculations was invented in the years 1975-78 that an efficient computational procedure for CASSCF calculations could be developed ". ... 

Roos, B. O., The complete active space self-consistent-field method and its applications in electronic structure calculations. Adv. Chem. Phys. 69 399 (1987). One of the most successful methods for finding highly accurate wave functions. CASSCF is a variant of MCSCF and uses all the tricks of the trade (direct Cl, unitary group approach, etc.) in its implementation. 

Brooks, B. R., 8c Schaefer, H. R, III. (1979). The graphical unitary group approach to the electron correlation problem. Methods and preliminary applications. The Journal of Chemical Physics, 70, 5092-5106. 

Saxe et al., 1982] P. Saxe, D. Fox, H. Schaefer, and N. Handy. The Shape-driven Graphical Unitary Group Approach to the Electron Correlation Problem. Application to the Ethylene Molecule, Journal of Chemical Physics 77 5584-5592, 1982. 

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