Hydrogen dynamics in solid formic acid: insights from simulations with quantum colored-noise thermostats

Abstract

Abstract. With an increase of computational capabilities, ab initio molecular dynamics becomes the natural choice for exploring the nuclear dynamics of solids. As based on classical mechanics, the validity of this approach is, in-principle, limited to the high-T regime, whilst low-temperature simulations require inclusion of quantum effects. The methods commonly used to account for nuclear quantum effects are based on the path-integral formalism, which become, however, particularly time consuming when high accuracy methods are used for calculating forces. Recently, new efficient alternative approaches to account for quantum nature of nuclei have been proposed, using so-called quantum thermostats. In this work, we examine the simulations performed with the quantum colored-noise thermostat introduced by Ceriotti [Phys. Rev. Lett., 103:030603, 2009]. We present the tests of portable implementation of the quantum thermostat in the ABIN program, which has been extended to periodic systems through the interface to CASTEP, a leading spectroscopy-oriented plane-wave density functional theory code. The range of applicability of quantum-thermostatted molecular dynamics simulations for the interpretation of neutron scattering data was examined and compared to classical molecular dynamics and lattice-dynamics simulations, using solid formic acid case as a test bed. We find that the approach is particularly useful for the modeling of low-temperature inelastic neutron scattering spectra as well as provides some theoretical estimate for the low-limit of the mean kinetic energy. While finding the quantum-thermostat to seriously affect the dynamic properties of the title system, we illustrate to which extent the unperturbed response can be successfully recovered. IntroductionMolecular dynamics (MD) has evolved into a powerful numerical method to investigate structural and dynamical properties of condensed matter [1]. For an atom, these calculations are, however, only valid in the classical limit, i.e. for temperatures safely above the Debye temperature that signals the onset of quantum effects. The Debye temperature is often of the order of few hundred Kelvins and nuclear quantum effects (NQEs) are thus indeed important over a wide range of temperatures [2,3].Quantum fluctuations stemming from the Heisenberg uncertainty principle make the energy of a particle at 0 K higher than a the potential energy minimum, leading to a concept of zero point energy (ZPE) (see Fig. 1). While ZPE reflects the quantum nature of atomic motion, it can be approximately accounted for by lattice-dynamics calculations, which, by definition, are based on the quantum harmonic oscillator model.

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