Visualizing Correlation Regions: The Case of the Ammonia Crystal

We resort to X-ray constrained wavefunctions in order to separately analyze crystal field and correlation effects in ammonia. With this aim in mind, we compare the electron density and the amount of electron localization in different molecular regions derived from three different calculations: the isolated molecule, the solid-state and the X-ray constrained wavefunctions. While the crystal field effects lead to a contraction (localization) of electron pairs, the introduction of correlation as a correction from the experimental density leads to a compensating effect that diffuses back again the electron pairs. We have also compared the effect on the most widely used methods in solid state, HF and DFT, showing that (as in molecules) correlation has well differenciated effects, with DFT overdelocalizing. It is now well known that approximate functionals have errors in the density reconstruction and in the energy estimation. Resorting to experimental densities thus allows expanding the separation of the errors in the functional and the density in solid state, where correlated wavefunctions are not easily available.