Ion-orbital coupling in Car-Parrinello calculations of hydrogen-bond vibrational dynamics: case study with the NH3-HCl dimer.

We have performed Car-Parrinello molecular dynamics (CPMD) calculations of the hydrogen-bonded NH(3)-HCl dimer. Our main aim is to establish how ionic-orbital coupling in CPMD affects the vibrational dynamics in hydrogen-bonded systems by characterizing the dependence of the calculated vibrational frequencies upon the orbital mass in the adiabatic limit of Car-Parrinello calculations. We use the example of the NH(3)-HCl dimer because of interest in its vibrational spectrum, in particular the magnitude of the frequency shift of the H-Cl stretch due to the anharmonic interactions when the hydrogen bond is formed. We find that an orbital mass of about 100 a.u. or smaller is required in order for the ion-orbital coupling to be linear in orbital mass, and the results for which can be accurately extrapolated to the adiabatic limit of zero orbital mass. We argue that this is general for hydrogen-bonded systems, suggesting that typical orbital mass values used in CPMD are too high to accurately describe vibrational dynamics in hydrogen-bonded systems. Our results also show that the usual application of a scaling factor to the CPMD frequencies to correct for the effects of orbital mass is not valid. For the dynamics of the dimer, we find that the H-Cl stretch and the N-H-Cl bend are significantly coupled, suggesting that it is important to include the latter degree of freedom in quantum dynamical calculations. Results from our calculations with deuterium-substitution show that both these degrees of freedom have significant anharmonic interactions. Our calculated frequency for the H-Cl stretch using the Becke-exchange Lee-Yang-Parr correlation functional compares reasonably well with a previous second-order Møller-Plesset calculation with anharmonic corrections, although it is low compared to the experimental value for the dimer trapped in a neon-matrix.