Accelerated path integral methods for atomistic simulations at ultra-low temperatures

Uhl, Felix; Marx, Dominik; Ceriotti, Michele

doi:10.1063/1.4959602

Cited by 34 publications

(32 citation statements)

References 53 publications

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“…Ab initio path integral molecular dynamics simulations were performed at 1.25 K. All electrons were treated explicitly at the BLYP/aug-cc-pVTZ level, as implemented in the CP2K code ( 50 ). The path integral was represented using P = 64 beads (also known as Trotter replica), which enforces convergence of the quantum discretization in conjunction with using PIGLET thermostatting in CP2K, as developed and benchmarked recently for ultralow temperatures ( 51 ).…”

Section: Methodsmentioning

confidence: 99%

Acid solvation versus dissociation at “stardust conditions”: Reaction sequence matters

et al. 2019

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Chemical reactions at ultralow temperatures are of fundamental importance to primordial molecular evolution as it occurs on icy mantles of dust nanoparticles or on ultracold water clusters in dense interstellar clouds. As we show, studying reactions in a stepwise manner in ultracold helium nanodroplets by mass-selective infrared (IR) spectroscopy provides an avenue to mimic these “stardust conditions” in the laboratory. In our joint experimental/theoretical study, in which we successively add H2O molecules to HCl, we disclose a unique IR fingerprint at 1337 cm−1 that heralds hydronium (H3O+) formation and, thus, acid dissociation generating solvated protons. In stark contrast, no reaction is observed when reversing the sequence by allowing HCl to interact with preformed small embryonic ice-like clusters. Our ab initio simulations demonstrate that not only reaction stoichiometry but also the reaction sequence needs to be explicitly considered to rationalize ultracold chemistry.

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Section: Methodsmentioning

confidence: 99%

Acid solvation versus dissociation at “stardust conditions”: Reaction sequence matters

et al. 2019

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“…This electronic structure setup has been shown to describe many properties of water in close agreement to experiment [60][61][62] . In total 20 ps (after an equilibration period of 1.5 ps using a starting configuration obtained from a well-equilibrated q-TIP4P/F force field simulation) were generated for analyses where the path integral has been discretized in terms of 48 replicas in conjunction with the PIGLET algorithm 39,53 All reported properties were evaluated for hydrogen bonded configurations, where the standard hydrogen bond criterion based on a donor-acceptor distance of 3.5Å and a hydrogen bond angle ∠ HOO of 30 • has been applied 63 . Other hydrogen bond criteria were explicitly tested but resulted only in minor differences with respect to the reported results in agreement with earlier systematic studies on this topic 64 .…”

Section: Computational Detailsmentioning

confidence: 99%

Quantum nature of the hydrogen bond from ambient conditions down to ultra-low temperatures

Schran

Marx

2019

Phys. Chem. Chem. Phys.

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Many experimental techniques such as tagging photodissociation and helium nanodroplet isolation spectroscopy operate at very low temperatures in order to investigate hydrogen bonding. To elucidate the differences between such ultra-cold and usual ambient conditions, different hydrogen bonded systems are studied systematically from 300 K down to about 1 K using path integral simulations that explicitly consider both, the quantum nature of the nuclei and thermal fluctuations. For that purpose, finite sized water clusters, specifically the water dimer and hexamer, protonated water clusters including the Zundel and Eigen complexes, as well as hexagonal ice as a condensed phase representative are compared directly as a function of temperature. While weaker hydrogen bonds, as present in the neutral systems, show distinct structural differences between ambient conditions and the ultra-cold regime, the stronger hydrogen bonds of the protonated water clusters are less perturbed by temperature compared to their quantum ground state. In all studied systems, the quantum delocalization of the nuclei is found to vary drastically with temperature. Interestingly, upon reaching temperatures of about 1 K, the spatial quantum delocalization of the heavy oxygens approaches that of the protons depending on the environment, and even significantly exceeds the latter in case of the centered hydrogen bond in the Zundel complex. These findings are relevant for comparisons between experiments on hydrogen bonding carried out at ultra-cold versus ambient conditions as well as to understand quantum delocalization phenomena of nuclei by seamlessly extending our insights into noncovalent interactions down to ultra-low temperatures.

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“…While these methods show great promise for accurate determination of spectroscopic properties [23][24][25], their cost remains high when combined with a potential energy surface computed by ab initio electronic structure methods.Among the many methods that have been introduced in the past decade to accelerate the convergence of path integral calculations [26], those that combine path integral molecular dynamics with a generalized Langevin equation [27][28][29] can be applied transparently to empirical, machine learning or first principles simulations. They have been used to evaluate all sorts of thermodynamic properties, including structural observables [30], free energies [31], momentum distributions [28], and quantum kinetic energies [32] with a reduction in computational effort varying between a factor of 5 at ambient conditions to a factor of 100 at cryogenic temperatures [33]. The aggressive thermostatting used to impose quantum fluctuations, however, significantly disrupts the dynamics of the system, and common wisdom is that the calculation of dynamical properties using PIGLET is impossible.Here we present a simple post-processing strategy that makes it possible to reconstruct dynamical properties from trajectories generated using PIGLET, leading to a dramatic reduction in the cost of including quantum effects in spectroscopic quantities.…”

mentioning

confidence: 99%

“…Among the many methods that have been introduced in the past decade to accelerate the convergence of path integral calculations [26], those that combine path integral molecular dynamics with a generalized Langevin equation [27][28][29] can be applied transparently to empirical, machine learning or first principles simulations. They have been used to evaluate all sorts of thermodynamic properties, including structural observables [30], free energies [31], momentum distributions [28], and quantum kinetic energies [32] with a reduction in computational effort varying between a factor of 5 at ambient conditions to a factor of 100 at cryogenic temperatures [33]. The aggressive thermostatting used to impose quantum fluctuations, however, significantly disrupts the dynamics of the system, and common wisdom is that the calculation of dynamical properties using PIGLET is impossible.…”

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confidence: 99%

Inexpensive modeling of quantum dynamics using path integral generalized Langevin equation thermostats

Kapil

Wilkins

Lan

et al. 2020

The Journal of Chemical Physics

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The properties of molecules and materials containing light nuclei are affected by their quantum mechanical nature. Modelling these quantum nuclear effects accurately requires computationally demanding path integral techniques. Considerable success has been achieved in reducing the cost of such simulations by using generalized Langevin dynamics to induce frequency-dependent fluctuations. Path integral generalized Langevin equation methods, however, have this far been limited to the study of static, thermodynamic properties due to the large perturbation to the system's dynamics induced by the aggressive thermostatting. Here we introduce a post-processing scheme, based on analytical estimates of the dynamical perturbation induced by the generalized Langevin dynamics, that makes it possible to recover meaningful time correlation properties from a thermostatted trajectory. We show that this approach yields spectroscopic observables for model and realistic systems which have an accuracy comparable to much more demanding approximate quantum dynamics techniques based on full path integral simulations.Vibrational spectroscopic techniques -from conventional infrared (IR) and Raman to advanced femtosecond pump-probe [1, 2], sum-frequency generation, second harmonic scattering [3, 4], and multi-dimensional vibrational spectroscopy [5] -are a cornerstone of chemistry [6]. These techniques have a multitude of applications such as the characterization of functional groups in chemical systems [7], the determination of the atomistic mechanisms of phase transitions through insight into chemical environments [8], and identification of unique structural fingerprints of molecular crystals [9]. The use of atomistic simulations for the computation of spectroscopic properties facilitates the interpretation of these experiments and provides support to the characterization of novel materials.Even neglecting effects that go beyond the Born-Oppenheimer (BO) decoupling of electronic and nuclear degrees of freedom, accurate calculations of the vibrational spectra of materials require an explicit treatment of the quantum dynamics of the nuclear degrees of freedom [10] on the electronic ground state potential energy surface. Quantum dynamics is in principle exactly obtained from the solution of the time dependent Schrödinger equation for the nuclei, but this is only practical for systems containing a handful of degrees of freedom [11,12]. Condensed phase systems can be studied [13] either by an exact treatment of the quantum dynamics of a subset of the nuclear degrees of freedom [14], or through classical dynamics on the quantum free energy surface of the nuclei [15,16]. Among the methods in the latter class, several of the most popular ones are based on the imaginary time path integral framework -such as (thermostatted) ring polymer molecular dynamics [17,18] ((T)RPMD), centroid molec- * venkat.kapil@epfl.ch ular dynamics [19,20] (CMD) and the recently developed quasi-centroid molecular dynamics [21] (QCMD). These methods ignore real time cohe...

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Accelerated path integral methods for atomistic simulations at ultra-low temperatures

Cited by 34 publications

References 53 publications

Acid solvation versus dissociation at “stardust conditions”: Reaction sequence matters

Acid solvation versus dissociation at “stardust conditions”: Reaction sequence matters

Quantum nature of the hydrogen bond from ambient conditions down to ultra-low temperatures

Inexpensive modeling of quantum dynamics using path integral generalized Langevin equation thermostats

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