Large amplitude motions of isolated Urea are analyzed using CCSD(T) calculations. Urea shows two conformers of C-s and C-2 symmetries that interconvert through the NH2 torsion and the NH2 inversion motions. Anharmonic frequencies are computed using second order perturbation theory and an ab initio force field. The potential energy surface shows 16 minima. Nine stationary points are determined. Five reduced Hamiltonians are used for the vibrational energy determination, concluding that at least a variational treatment based on a 4D model is required for the FIR spectrum simulation. The inversion barrier produces a large splitting of the vibrational ground state. (C) 2011 Elsevier B. V. All rights reserved.