Correlation energy natural definition

X h is an eigenstate of H with eigenvalue n o>. 4 il assumed to be of the " CC-form. In the presence of interaction, the eigenstate of the composite system will not have a definite number of photons, and the extent of correlation as a result of coupling will also be modified. These changes can be induced by the action of a second cluster operator of the exponential form exp(S). The operator S destroys/creates zero, one, two,..., photons and simultaneously induce various nh-mp excitations out of 3 . The nature of the electronic part of the cluster operator in S is dictated by the nature of the energy difference we are interested in. For IP/EA calculations, V in eq. (5.3.1) will destroy/create an electron from should involve nh-(n+l)p excitations. Similarly, for EE, V will conserve the number of electrons, and S should involve nh-np excitations. For computing the linear response, it suffices to retain only the terms linear in C/CT ... 

The empirical nature of is obvious, and it would be helpful to replace y by parameters having a sound basis in thermodynamic or statistical mechanical considerations. Recent efforts by Fowkes [40] to relate y to the dispersion forces between molecules at the interface have been especially promising in leading to tractable equations. An interesting direct correlation has been recently pointed out to us by Gar don [49] between the value of y of a solid polymer and the Hildebrand solubility parameter, 6, which is defined as the square root of the molar energy density—i.e., 6 = n/1E/p A simple consideration of the Young equation and the definition of y indicates that when cos 0=1,... 

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