Multideterminant wavefunction

This sum is over the n occupied MOs ij/t for a closed-shell molecule, for a total of 2n electrons. Equation 7.1 applies strictly only to a single-determinant wave-function T, but for multideterminant wavefunctions arising from configuration interaction treatments (Section 5.4) there are similar equations [10]. A shorthand for p(x, y, z) dxdydz is p(r)dr, where r is the position vector of the point with coordinates (x, y, z). 

Although a spin-orbital formulation is conceptually simple, desirable properties such as spin-adaptation may be lost when the electronic state of interest is open shell, for example. A rigorously spin-adapted theory must include spin-free definitions of the cluster operators, T, and an appropriate (perhaps multideterminant) reference wavefunction (Refs. 39, 41, 42, 156-158). Such general coupled cluster derivations are beyond the scope of this chapter, though some of the issues associated with difficult open-shell problems are discussed in the next section. 

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