Improved descriptions of single-electron processes require treatments beyond the geometric approximation given by the Hartree-Fock electron propagator in Eq. (4.25). More elaborate approximations of the equation of motion (9.1)

may be generated in various ways. The discussion here will be confined to the use of perturbation theory.

Such a treatment can, with advantage, be expressed in terms of the superoperators introduced in Eq. (4.19) and in terms of a basis of field operators. The basis of fermion-like operators {*X _{i}*} = is chosen, such that the electron field operators correspond to the SCF spin orbitals. The field operator space supports a scalar product (

Perturbation theory starts with a partitioning of the hamiltonian, and thus ...

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