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It is generally accepted that nuclear quantum effects (NQEs) trigger the transition to the nonmolecular form of ice under increasing pressure. This picture is challenged in salty ice, where Raman scattering measurements up to 130 GPa of molecular ice VII containing NaCl or LiCl impurities show that the transition pressure to the symmetric phase ice X is shifted up by about 30 GPa, even at small salt concentrations. We address the question of how the inclusion of salt induces the drastic reduction of NQEs by selectively including NQEs in ab initio calculations of ice in the presence of distinct ionic impurities. We quantitatively show that this is mainly a consequence of the electric field generated by the ions. We propose a simple model that is able to capture the essence of this phenomenon, generalizing this picture to other charged defects and for any concentration. This result is potentially generalizable to most "dirty" ices in which the electric field due to the doping is much more significant than local lattice distortions. [ABSTRACT FROM AUTHOR]

The phase transition from ice VII to ice X under extreme pressures is an example where quantum protondelocalization coexists with classical thermal fluctuations. We investigate this transition, including quantumeffects on the nuclear motion through adapted Langevin dynamics. This approach, which allows us to followthe semiclassical trajectories of protons, provides excellent agreement with experimental vibrational spectraindicating a transition pressure of about 65 GPa. Furthermore, we map the full dynamical problem onto apressure-dependent, one-dimensional mean-field potential for the proton. By solving exactly the corresponding Schödinger equation, we disentangle tunneling and quantum delocalization from classical thermal effects andidentify the transition through the topological changes of the proton ground state and its susceptibility. Theprocess is dominated by quantum effects even at ambient temperature and can be considered to be a paradigmaticcase of a quantum-driven phase transition. [ABSTRACT FROM AUTHOR]

The quantum thermal bath (QTB) has been presented as an alternative to path-integral-based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well-known problem in methods based on classical trajectories where part of the energy of the high-frequency modes is transferred to the low-frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce and, in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well-reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.