Dynamic electron correlation energy orbitals

Insofar as the latter process docs not involve any orbital reoptimization for any particular state, it provides a wave function that is roughly equivalent in quality only to an HF wave function for the ground state. Of course, this may still be useful for a number of purposes. CIS results for six excited states of benzene are included in Table 14.2, as are results from other levels of theory that will be discussed later. The CIS results are qualitatively useful, insofar as the states are correctly ordered, and the error is fairly systematic - all states are predicted to be too high in energy by an average of 0.7 eV. The worst prediction is for the lowest excited state, which is known to have significant dynamical electron correlation, and is therefore challenging for the CIS method. 

The dynamic electron correlation means the rest of the correlation effect, beyond the static one. It corresponds also to occupying orbital energies, but not those related to the degeneracy or quasi-degeneracy of the ground state. As we see, the distinction between the static and the dynamic correlation is a bit arbitrary. 

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