Correlation with bond-length alternation

Table 19 The bond-length alternation parameter (in A) for the push-pull oligomers of Fig. 8. The phase shows whether the calculations were performed in vacuum or in the water phase, and HF and MP2 mark Hartree-Fock calculations without (HF) or with (MP2) the inclusion of correlation effects via Moller-Plesset perturbation theory. All results are from ref. 70... 

At the HF level, the value of the C=C bond length is clearly underestimated. The inclusion of electron correlation at different levels of calculation leads to values in closer agreement with experiment. The value of the C—C bond length is less sensitive to the inclusion of electron correlation. As a consequence of this fact, the CC bond alternation (the difference between CC single and double bond lengths) is overestimated at the HF level. The inclusion of dynamical electron correlation through MP calculations corrects this error. A very similar result is obtained at the CASSCF level of calculation31. 

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