A technique for the configuration interaction (CI) study of many-electron systems is developed based on Rumer spin-coupling scheme for the antisymmetrized configuration state functions (CSF). Incorporating a new graphical approach, the primitive configurations have been generated in blocks of definite ionocities to permit ready association of possible spin functions with each of the primitive configurations. Simple as well as extended Hubard model Hamiltonians have been studied to test the efficiency of the method. Procedures have been incorporated to calculate various correlation functions using the spin-adapted CSFs without invoking explicit expansions in terms of slater determinants. 0 1996 John Wiley & Sons, Inc.
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(UGA) are basically of two types. They are the graphical unitary group approaches (GUGA) [1,21 and the permutation group based approach [3,41. In the former, the U ( n ) shift operator, E,,, is expressed in terms of elementary shift operators, E, I + through a commutator bracket expansions and in the latter the matching permutation P E S, is broken into elementary transpositions ( k , k + 1) in order to use simple Young rules for matrix representations.
In addition to these two possibilities, Prof. McWeeny has been a strong advocate of the use of linearly independent Rumer spin-coupled functions to carry out CI studies . The main advantage of this approach is that we need only simple rules for determining matrix elements of the spinfree Hamiltonian based on the counting of islands, e-chains and o-chains in the superposition dia-