Isotope effects of ammonia umbrella flip using semiclassical instanton calculations based on discretized path integrals

We have recently developed a new algorithm to calculate tunneling splittings for molecular systems using the discretized path integral instanton method Kawatsu and Miura (2014) [29]. Compared with the standard method, the new algorithm enables us to reduce the computational cost without decreasing the numerical accuracy. The algorithm has been combined with ab initio electronic structure calculations on the fly. In this Letter, we apply our method to ammonia umbrella flips with five different isotopologues: ND3, NT3, (NH3)-N-15, (ND3)-N-15, NH2D and NHD2. The calculated tunneling splittings are found to be in quantitative agreement with experimental data in all of these calculations as well as for NH3. The method provides a useful tool for evaluating tunneling splittings for such rigid molecular systems. (C) 2015 Elsevier B.V. All rights reserved.