Solids modeled by ab-initio crystal field methods. Effects of intermolecular interactions on the vibrational spectrum of urea

In this study (Part 18 in the series entitled "Solids modeled by ab-initio crystal field methods") the effect of intermolecular coupling upon the vibrational spectrum of urea in its P421m crystal phase is calculated in the harmonic approximation and compared with experimental infrared and Raman frequencies determined at room temperature. Obtained crystal splittings range from about 5 cm−1 (7 cm−1 experimental, 5 cm−1 calculated) to about 60 cm−1 (61 cm−1 experimental, 59 cm−1 calculated). In comparison to a previous study (Rousseau et al., J. Phys. Chem. A 102 (1998) 6540) use of intermolecular coupling improved agreement between theory and experiment. Calculations use the molecular cluster model at the RHF/6-31++G∗∗ level of theory.