Thus, the A 1 term specifically denotes the one-particle fluctuations in the virtual space upon double excitation. Viewing this contribution as a set of time-ordered operations and looking at only one choice of time ordering, the effect of the A 1 operator can be described as beginning with a single excitation i → a, subsequent coupling of a to b via the fock matrix, followed by a deexcitation b → j. In general, these anti-Hermitian terms represent fluctuations of single particles or holes through specific time-ordered excitation, coupling and deexcitation processes. For example, A 5 can denote an excitation followed by deexcitation (i → a → k), after which the particle is coupled via the Fock matrix and again excited and deexcited (l → b → j). Related observations can be made about B 1 , A 2 and B 2 . We note that their algebraic form does not appear in a CC derived expression and can therefore be referred to as fictitious. For this reason, an educated ab initio guess for their optimal parameters can hardly be made.