Global analytical potential energy surface for large amplitude nuclear motions in ammonia

An analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state is developed, and parameters are determined by adjustment to ab initio data and thermochemical data for several low-lying dissociation channels. The electronic structure is calculated at the CASPT2 level within an [8,7] active space. The representation is compared to other recently published potential energy surfaces for this molecule. The present representation is distinguished by giving a qualitatively correct description of the potential energy for very large amplitude displacements of the nuclei from equilibrium. Other characteristic features of the present surface are the equilibrium geometries r(eq)(NH(3)) approximately 101.24 pm, r(eq)(NH(2)) approximately 102.60 pm, alpha(eq)(NH(3)) approximately 106.67 degrees, and the inversion barrier at r(inv)(NH(3)) approximately 99.80 pm and 1781 cm(-1) above the NH(3) minimum. The barrier to linearity in NH(2) is 11,914 cm(-1) above the NH(2)((2)B(1)) minimum. While the quartic force field for NH(3) from the present representation is significantly different from that of the other potential energy surfaces, the vibrational structures obtained from perturbation theory are quite similar for all representations studied here.