The PCM-CC-PTE Approximation

The explicit equations for the PCM-CC-PTE approximation can be obttiined from the PCM-CCSD-PTED equations by neglecting all the explicit terms involving the correlation components of the solvent reaction potential.The resulting PTE-PCM-CCSD equations have the same form of the CCSD equations for an isolated molecule, with the only difference that now the Fock matrix elements and the MO refer to those of the solvated molecules. 

If in the ground state of the molecular solute the electron-correlation contribution to the solute-solvent interaction is neglected (i.e. in the PCM-CC-PTE approximation of Sect. (1.2.1)), the LR-CC excitation energies are ... 

The PCM coupled-cluster (PCM-CC) theory [9-13] introduces an explicit description of the coupling between the electron-correlation (dynamic) of the molecular solute and the solute reaction potential. The electron-correlation modifies the charge distribution Pm of the solute. The changes in charge distribution pm modify the solvent reaction potential Va, which in turn influence the electron-correlation. If the coupling between the dynamical electronic correlation of the solute and the polarization of the solvent is neglected the dynamic electron-correlation of the molecular solutes is evaluated in the presence of the fixed Hartree-Fock solvent reaction potential. This approximated form of the PCM-CC theory is denoted with the acronym PTE (i.e. Perturbation Theory on the Energy) which derives from a many-body perturbation analysis of the solute-solvent interaction [14]. 

In the PCM-EOM-CC/SACCI approximation the excited electronic states are represented by a linear (Cl-like) expansion build-up on the coupled-cluster wavefunction for the ground state [6-8]. We use the PTE couple-cluster wavefunction, computed in the presence of the frozen Hartree-Fock reaction field, as it leads to a more simpler and physically transparent PCM-EOM theory. The EOM-CC theory leads to a non-Hermitian eigenvalue problem with right and left eigenvalues. 

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