DENSITY-FUNCTIONAL STUDY OF THE STRUCTURES AND NONLINEAR-OPTICAL PROPERTIES OF UREA

The geometry, conformations, and vibrational spectra of urea have been predicted at the density functional theory level (DFT). The molecule is predicted to have a lowest energy C-2 conformer but with very floppy NH2 groups. The torsion barrier about the C-N bond is predicted to be about 7 kcal/mol. Calculations on model compounds were also performed in order to better understand the dependence of the structural predictions on the level of DFT employed. The dipole moments, polarizabilities, and first- and second-order hyperpolarizabilities of urea were calculated and found to be in good agreement with the available experimental values. The nonlinear optical properties of the small molecules H-2, N-2, CO, CH4, NH3, H2O, HF, and formaldehyde have also been calculated including gradient (nonlocal) corrections. This shows that gradient corrections make only a small contribution to these properties and that the inclusion of diffuse functions is more important.