Are intramolecular dynamic electron correlation effects detectable in X-ray diffraction experiments on molecular crystals?

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Abstract
In order to assess whether the effects of intramolecular dynamic electron correlation on the electron density would be experimentally detectable, X-ray structure factors which include thermal averaging effects have been calculated from the electron densities of a range of small-molecule molecular crystals [C2H6, C2H4, C2H2, BH3NH3, NH3, NH2CN, OCl2, CO(NH2)(2)] using the procrystal, Hartree-Fock, B3LYP and QCISD wavefunction models with the superposition-of-independent-molecules method to create the electron density in the crystal. A naive R-factor-like criterion of 1% has been used to assess detectability, as well as a more sophisticated method based on real X-ray data for estimating experimental errors. Correlation effects on the density are found to be only marginally above the 1% detectability threshold, and are about one to two orders of magnitude smaller than deviations from the procrystal model. Further, only 10% of the data up to 1.2 angstrom(-1) are significant for detecting correlation effects; and of those 10%, many are at low intensity and therefore difficult to measure. Another method to estimate the experimental errors indicates that the intramolecular correlation effects would not be measurable. Although thermal averaging effects are important for the absolute value of the calculated structure factors, the use of different thermal averaging models does not change our overall conclusion of detectability. Likewise, calculations using the B3LYP method for some molecules do not show significant changes in the amount of, or distribution of, the changes that would be detectable by experiment.