MP2 versus density-functional theory study of the Compton profiles of crystalline urea

The Compton profiles (CP) of crystalline urea are computed ab initio at different levels of theory and compared with accurate experimental measurements. The CRYSTAL program is used in order to collect the Hartree-Fock (HF) and density-functional theory (DFT) results, while the new CRYSCOR code is adopted for the calculation of the MP2 correction to the HF density matrix. It is demonstrated that the role of electron correlation (Fermi and Coulomb) is crucial in predicting the correct CPs; DFT is shown to provide results in partial disagreement with the experiment, at variance with the HF/MP2 treatment that correctly predicts the CP anisotropies of urea. We demonstrate that the global effect of dynamic electron correlation is the reduction of the anisotropy of the electronic momentum distribution within the crystal.