CASPT theory

Although not rigorously size-extensive, CASPT behaves better than MRSDCI theory for systems of many electrons. The CASPT and other MRPT methods are important in the sense that they are the only generally applicable methods for ab initio calculations of dynamical correlation of open-shell and closed-shell multiconfigura-... 

To set up a size-extensive perturbation theory, the zero-order Hamiltonian must be additively separable. We shall here show that, for multiplicatively separable zero-order wave functions, the CASSCF Fock operator and the zero-order energy are both additively separable but that the presence of projection operators nevertheless makes the zero-order CASPT Hamiltonian H ... 

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