| Abstract
| - The structure and cohesive energy of crystalline urea have been investigated at the ab initio level of calculation.The performance of different Hamiltonians in dealing with a hydrogen-bonded molecular crystal as crystallineurea is assessed. Detailed calculations carried out by adopting both HF and some of the most popular DFTmethods in solid-state chemistry are reported. Local, gradient-corrected, and hybrid functionals have beenadopted: SVWN, PW91, PBE, B3LYP, and PBE0. First, a 6-31G(d,p) basis set has been adopted, and thenthe basis set dependence of computed results has been investigated at the B3LYP level. All calculations werecarried out by using a development version of the periodic ab initio code CRYSTAL06, which allows fulloptimization of lattice parameters and atomic coordinates. With the 6-31G(d,p) basis set, structural featuresare well reproduced by hybrid methods and GGA. LDA gives lattice parameters and hydrogen-bond distancesthat are too small relative to experiment, while at the HF level the opposite trend is observed. Results showthat hybrid methods are more accurate than HF and both LDA and GGA functionals, with a trend in thecomputed properties similar to that of hydrogen-bonded molecular complexes. When BSSE and ZPE aretaken into account, all methods, except LDA, give computed cohesive energies that are underestimated withrespect to the experimental sublimation enthalpy. Dispersion energy, not properly taken into account by DFTmethods, plays a crucial role. Such a deficiency also affects dramatically the computed crystalline structure,especially when large basis sets are adopted. We show that this is an artifact due to the BSSE. Indeed, withsmall basis sets the BSSE gives an extra-binding that compensates for the missing dispersion forces, thusyielding structures in fortuitous agreement with experiment.
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