CASVB algorithm

All of the key features of the CASVB algorithms have been described in detail elsewhere [1-7], and so we present here a brief overview. We write a normalized CASSCF wave function in the form... 

The CASVB strategy [1-9] uses a very efficient algorithm for the transformation of CASSCF [10] structure spaces, for the interpretation of CASSCF wavefunctions, and for the fully-variational optimization of VB wavefunctions. Important features for the quality of the final description include the unbiased optimization of both the VB orbitals and the mode of spin coupling, and also flexibility in the choice of the form of wavefunction. 

Some of the most efficient algorithms currently available for spin-coupled calculations involve the expansion of the SC wavefunction in terms of Slater determinants, leading to a summation over (N-p) -dimensional cofactors for the p-particle density matrix [23]. For calculations involving the full spin space, a further saving can be achieved by the use of projected spin functions [24], An alternative strategy is provided by CASVB (see Sect. 5). 

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