The results of many-body analysis of the intermolecular interaction energy for the selected subsystems of the molecular crystal of urea are discussed. In particular, a great importance of cooperative effects in both, the interaction energy and linear and non-linear optical properties are considered. The nature of intermolecular interactions in the urea clusters is investigated applying the hybrid variational-perturbational interaction energy decomposition scheme. Molecular polarizabilities and first-order hyperpolarizabilities are estimated by means of a finite-field technique combined with the Hartree-Fock and the second-order Moller Plesset perturbation theories. The results of our calculations indicate a substantial influence of the many-body effects on the non-linear response of the studied hydrogen-bonded systems. (c) 2005 Elsevier B.V. All rights reserved.