Simulation
(mai 2019)
13. Isotope effect on hydrogen bond symmetrization
in hydrogen and deuterium fluoride crystals by molecular dynamics simulation
H. Dammak, F.
Brieuc, G. Geneste, M.
Torrent, M. Hayoun, Phys. Chem. Chem. Phys. 21 (2019) pp.3211-3217 Paper
DOI: http://dx.doi.org/10.1039/c8cp06949b
The isotope effect on the collective
proton/deuteron transfer in hydrogen and deuterium fluoride crystals has been
investigated at 100 K by ab initio quantum-thermal-bath path-integral molecular
dynamics (QTB-PIMD) simulation. The deuterons within a planar zigzag chain of
the orthorhombic structure simultaneously flip between covalent and hydrogen
bonds due to the barrier crossing through tunnelling.
The height of the corresponding static barrier normalized for one deuteron is
29.2 meV. In the HF crystal, all the protons are
located at the center of the heavy-atom distance. This evidences the symmetrization of the H-bonds, and
indicates that the proton zero-point energy is above the barrier top. The
decrease of the heavy-atom distance due to quantum fluctuations in both HF and
DF crystals corresponds to a large decrease and an increase of the hydrogen and
covalent bond lengths, respectively. Upon deuteration, the increase of the
heavy-atom distance (Ubbelohde effect) is in agreement with experimental data.
12. Nuclear quantum effects in molecular dynamics simulations
H. Dammak, M.
Hayoun, F Brieuc, G. Geneste, J. Phys.: Conf. Ser. 1136 (2018) 012014 Paper
DOI: http://dx.doi.org/10.1088/1742-6596/1136/1/012014
To take into account nuclear quantum effects on
the dynamics of atoms, the path integral molecular dynamics (PIMD) method used
since 1980s is based on the formalism developed by R. P. Feynman. However, the
huge computation time required for the PIMD reduces its range of applicability.
Another drawback is the requirement of additional techniques to access time
correlation functions (ring polymer MD or centroid MD). We developed an
alternative technique based on a quantum thermal bath (QTB) which reduces the
computation time by a factor of ~20. The QTB approach consists in a classical
Langevin dynamics in which the white noise random force is replaced by a
Gaussian random force having the power spectral density given by the quantum
fluctuation-dissipation theorem. The method has yielded satisfactory results
for weakly anharmonic systems: the quantum harmonic
oscillator, the heat capacity of a MgO crystal, and isotope effects in 7LiH
and 7LiD. Unfortunately, the QTB is subject to the problem of
zero-point energy leakage (ZPEL) in highly anharmonic
systems, which is inherent in the use of classical mechanics. Indeed, a part of
the energy of the high-frequency modes is transferred to the low-frequency
modes leading to a wrong energy distribution. We have shown that in order to reduce or even eliminate ZPEL, it is sufficient
to increase the value of the frictional coefficient. Another way to solve the
ZPEL problem is to combine the QTB and PIMD techniques. It requires the
modification of the power spectral density of the random force within the QTB.
This combination can also be seen as a way to speed up
the PIMD.
11. Proton diffusion mechanisms in the double perovskite
cathode material GdBaCo2O5.5: A molecular dynamics study
F. Brieuc, G.
Dezanneau, M. Hayoun and H. Dammak, Solid State
Ionics 309 (2017) pp
187–191 Paper DOI: http://dx.doi.org/10.1016/j.ssi.2017.07.017
GdBaCo2O5+x compounds have demonstrated to be very
efficient cathode materials not only in solid oxide fuel cells but also in
proton conducting fuel cells. In this last case, the excellent properties could
be due to the presence of mixed electron-proton conduction. We study here the
diffusion of the proton in this material using molecular dynamics simulations.
Two different diffusion mechanisms are observed. The predominant mechanism is
the standard proton transfer between two neighbouring
oxygen atoms combined with the rotation of H around its first neighbour oxygen atom. The second mechanism consists in the
migration of the OH group where both oxygen and hydrogen atoms diffuse
together. Strong spatial correlations between successive proton jumps are
evidenced. This is likely related to the presence of oxygen vacancies and to
the concerted diffusion of hydrogen and oxygen atoms.
10. Zero-point Energy Leakage in Quantum Thermal Bath
Molecular Dynamics Simulations:
F. Brieuc, Y.
Bronstein, H. Dammak, P. Depondt, F. Finocchi and M. Hayoun, J. Chem. Theory Comput.,
2016, 12 (12), pp 5688–5697 . Paper
DOI: 10.1021/acs.jctc.6b00684
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.
9. Quantum Thermal Bath for Path Integral Molecular
Dynamics Simulation:
F. Brieuc, H. Dammak and M. Hayoun, J.
Chem. Theory Comput. 12,
(2016) 1351-1359 . Paper DOI: 10.1021/acs.jctc.5b01146
The
quantum thermal bath (QTB) method has been recently developed to account for
the quantum nature of the nuclei by using standard molecular dynamics (MD)
simulation. QTB-MD is an efficient but approximate method when dealing with
strongly anharmonic systems, while path integral
molecular dynamics (PIMD) gives exact results but in a huge amount of
computation time. The QTB and PIMD methods have been combined in order to
improve the PIMD convergence or correct the failures of the QTB-MD technique.
Therefore, a new power spectral density of the random force within the QTB has
been developed. A modified centroid-virial estimator of the kinetic energy,
especially adapted to QTB-PIMD, has also been proposed. The method is applied
to selected systems: a one-dimensional double-well system, a ferroelectric
phase transition, and the position distribution of an
hydrogen atom in a fuel cell material. The advantage of the QTB-PIMD method is
its ability to give exact results with a more reasonable computation time for
strongly anharmonic systems.
8. Surface enhanced infrared absorption in dielectric
thin films with ultra-strong confinement effects:
Y. Chalopin, M. Hayoun, S. Volz and
H. Dammak, Applied Physics Letter, 104,
(2014) 011905 . Paper DOI: 10.1063/1.4860989
By
formulating a microscopic description of the non-local dielectric constant, we
have investigated the mechanisms of infrared absorption in dielectrics thin
films by molecular dynamics simulations. We found that light absorption in
dielectric slabs does not occur predominantly at the polaritons resonances but
through anomalous surface modes extremely confined in space. This demonstrates
that any macroscopic description of electrodynamics in dielectrics breaks down
at the nanoscale.
7. Low-temperature anharmonicity of barium titanate: A path-integral molecular-dynamics study:
G. Geneste, H. Dammak, M. Hayoun,
and M. Thiercelin, Physical Review B 87, (2013) 014113 (9pp) . Paper DOI: 10.1103/PhysRevB.87.014113
We
investigate the influence of quantum effects on the dielectric and
piezoelectric properties of barium titanate in its
(low-temperature) rhombohedral phase, and show the strongly anharmonic
character of this system even at low temperature. For this purpose, we perform
path-integral molecular-dynamics simulations under fixed pressure and fixed
temperature, using an efficient Langevin thermostat-barostat,
and an effective Hamiltonian derived from first-principles calculations. The
quantum fluctuations are shown to significantly enhance the static dielectric
susceptibility (≈ by a factor of 2) and the piezoelectric constants,
reflecting the strong anharmonicity of this ferroelectric system even at very
low temperature. The slow temperature-evolution of the dielectric properties
observed below ≈100 K is attributed (i) to
zero-point energy contributions and (ii) to harmonic behavior if the quantum
effects are turned off.
6. Isotope effects in lithium hydride and lithium
deuteride crystals by molecular dynamics simulations:
H. Dammak, E.
Antoshchenkova, M. Hayoun and F. Finocchi, J. Phys.: Condens.
Matter 24 (2012) 435402 (6pp). Paper :doi:10.1088/0953-8984/24/43/435402
Molecular
dynamics (MD) simulations have been carried out to study isotope effects in
lithium hydride and lithium deuteride crystals. Quantum effects on nuclear
motion have been included through a quantum thermal bath (QTB). The interatomic
forces were described either within the density-functional theory (DFT) in the
generalized gradient approximation (GGA) or by the phenomenological approach
using the shell model. For both models, the isotopic shift in the lattice
parameter can be successfully predicted by QTB-MD simulations. The slope of the
experimental isotopic shift in pressure is satisfactorily reproduced by QTB-MD
within DFT-GGA, in contrast to both density-functional perturbation theory and
QTB-MD with the shell model. We have analyzed the reasons of these discrepancies
through the vibrational densities of states and the isotopic shifts in bulk
modulus. The results illustrate the importance of anharmonic
contributions to vibrations and to the isotopic pressure shift between LiH and LiD.
5. Size dependent infrared properties of MgO
nanoparticles with evidences of screening effect:
Y. Chalopin, H. Dammak, M. Hayoun,
M. Besbes and J.J. Greffet, Applied Physics Letter,
100, (2012) 241904. Paper
We have investigated the infrared (IR) absorption properties of MgO
nanoparticles (NPs) with the means of molecular dynamics simulations. Several
size effects have been observed. We show in particular that the absorption of
IR radiation does not occur predominantly through the polariton mode but
preferentially through size dependent surface modes. This enhanced surface
absorption is found to result from a screening effect of the first atomic
layers of the NPs. We demonstrate concomitantly that a macroscopic description of
electrodynamics is inadequate to capture these unusual IR properties.
4. Radiative heat transfer from a black body to
dielectric nanoparticles:
Y. Chalopin, H. Dammak, M. Laroche, M. Hayoun,
and J.J. Greffet, Physical Review B 84,
(2011) 224301. Paper
Heating
of dielectric nanoparticles by black-body radiation is investigated by using
molecular-dynamics simulation. The thermal interaction with the radiation is
modeled by coupling the ions with a random electric field and including a
radiation reaction force. This approach shows that the heat is absorbed by the
polariton mode. Its subsequent redistribution among other vibration modes
strongly depends on the particle size and on temperature. We observe energy
trapping in a finite subset of vibrational modes and study the relaxation
pathway of (MgO)4 by performing a selective excitation with a
deterministic force.
3. Comment
on “Quantum thermal Bath for Molecular Dynamics simulations” Reply:
H. Dammak et al, Physical Review
Letters 107, (2011) 198902.
Paper
The PIMD method can provide
the exact position distribution even for anharmonic
systems. This method is very time consuming and therefore the range of its
applicability is reduced. The QTB-MD technique is an approximate approach that
yields accurate results and saves at least 2 orders of magnitude of computation
time compared to the PIMD method.
2. Quantum
Thermal Bath for Molecular Dynamics simulation:
H. Dammak et al, Physical Review
Letters 103, (2009) 190601.
paper
In this letter
we propose a method to account for the quantum nature of a thermal bath (QTB)
in standard classical molecular dynamics (MD) simulations. We use a
Langevin-type approach and introduce both a dissipative force and a random
force having the power spectral density given by the quantum
fluctuation-dissipation theorem. We apply the proposed method to compute the
mean energy of a gas of HD molecules, the heat capacity of a MgO crystal and
the self-diffusion coefficient of liquid Helium-4 at 2.5 K obtaining in all the
cases, good agreement with experiments in contrast with standard MD. We think
that the QTB method will significantly extend the domain of application of MD.
1. Cation order-disorder in Pb(BII,BV)O3-related relaxors: The Random-Layer Model investigated by Monte
Carlo Simulation:
H. Dammak and M. Hayoun,
Journal of Physics and Chemistry of Solids . 66(10) (2005)
1838-1843 (6 pages). paper
The
charge-balanced random-layer model for ordered lead-based perovskites Pb(BII1/3BV2/3)O3
was investigated by using the standard Metropolis Monte Carlo method on a rigid
lattice with simple ionic model. Our results show that in the structure formula
Pb[B’]1/2[B”]1/2O3, where all B”-sites are
occupied by BV cations, chemical order of BII and BV
cations does exist in B’-sites and the ordered structure has an hexagonal
symmetry. An order-disorder transition as a function of temperature is
evidenced by an abrupt variation of both the heat capacity and a long-range
order parameter. Finally, the evolution of the short-range order parameter
versus temperature shows that a local order remains in B’-sites contrary to the
charge-balanced random-layer model that suggests that B’-sites are randomly
occupied. This local order could be helpful to clarify some experimental
results.
Hichem DAMMAK