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 NH3 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(NH3) ≈ 101.24 pm, r eq(NH2) ≈ 102.60 pm, αeq(NH3) ≈ 106.67°, and the inversion barrier at r inv(NH3) ≈ 99.80 pm and 1781 cm-1 above the NH3 minimum. The barrier to linearity in NH2 is 11 914 cm-1 above the NH2(2B1) minimum. While the quartic force field for NH3 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.