Third-order polarization propagator approximation

Evaiuating aii the terms in the partitioned form of the poiarization propagator, Eq. (10.15), through third order one obtains the third-order polarization propagator approximation (TOPPA). The expressions for aii matrix eiements have been derived but oniy parts have been impiemented (Geertsen et at, 1991a). 

This keeps essentially the structure of the SOPPA equations but replaces in all matrix elements the first-order MP doubles correlation coefficients, Eq. (9.67), and the second-order MP singles correlation coefficient, Eq. (9.71), by coupled cluster singles and doubles amplitudes. In the earlier coupled cluster singles and doubles polarization propagator approximation (CCSDPPA) (Geertsen et al., 1991a), a precursor to SOPPA(CCSD), this was done only partially and in particular not in the second-order correction to the density matrix Very recently, a third method (Kjaer... 

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