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Researcher profile

Sebastien Hamel

Sebastien Hamel contributes to research discovery and scholarly infrastructure.

ResearcherAffiliation not importedOpen to collaborate
cond-mat.mtrl-sciphysics.comp-phphysics.chem-phphysics.plasm-phastro-ph.SRcond-mat.stat-mechMachine Learning

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Published work

9 published item(s)

preprint2026arXiv

Correlation function metrology for warm dense matter: Recent developments and practical guidelines

X-ray Thomson scattering (XRTS) has emerged as a valuable diagnostic for matter under extreme conditions, as it captures the intricate many-body physics of the probed sample. Recent advances, such as the model-free temperature diagnostic of Dornheim et al. [Nat.Commun. 13, 7911 (2022)], have demonstrated how much information can be extracted directly within the imaginary-time formalism. However, since the imaginary-time formalism is a concept often difficult to grasp, we provide here a systematic overview of its theoretical foundations and explicitly demonstrate its practical applications to temperature inference, including relevant subtleties. Furthermore, we present recent developments that enable the determination of the absolute normalization, Rayleigh weight, and density from XRTS measurements without reliance on uncontrolled model assumptions. Finally, we outline a unified workflow that guides the extraction of these key observables, offering a practical framework for applying the method to interpret experimental measurements.

0 citations0 reviewsNo code linkedOpen access
preprint2026arXiv

Polarizable atomic multipoles for learning long-range electrostatics

Long-range electrostatics and polarization remain central obstacles to extending machine learning interatomic potentials (MLIPs) to ionic, polar, and interfacial systems. Here, we introduce a semi-local framework for learning electrostatics from energies and forces using polarizable atomic multipoles. Local equivariant descriptors predict environment-dependent latent monopoles, dipoles, and quadrupoles, while residual non-local charge transfer and polarization are captured by non-self-consistent linear response in induced charges and dipoles. Across four diverse benchmarks and four short-range MLIP architectures, the multipole hierarchy and response terms systematically improve potential energy surface accuracy, with the largest gains in systems where long-range effects are essential. More importantly, the learned latent variables recover physically meaningful electrical responses: accurate Born effective charge tensors, emergent polarizabilities, infrared spectra in close agreement with experiments, and semi-quantitative Raman spectra for bulk water and hybrid MAPbI$_3$ perovskite. This systematically improvable, physically transparent framework enables MLIPs trained on standard energy and force labels to predict polarization-sensitive observables.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Dislocation-based strength model for high energy density conditions

We derive a continuum-level plasticity model for polycrystalline materials in the high energy density regime, based on a single dislocation density and single mobility mechanism, with an evolution model for the dislocation density. The model is formulated explicitly in terms of quantities connected closely with equation of state (EOS) theory, in particular the shear modulus and Einstein temperature, which reduces the number of unconstrained parameters while increasing the range of applicability. The least constrained component is the Peierls barrier $E_P$, which is however accessible by atomistic simulations. We demonstrate an efficient method to estimate the variation of $E_P$ with compression, constrained to fit a single flow stress datum. The formulation for dislocation mobility accounts for some or possibly all of the stiffening at high strain rates usually attributed to phonon drag. The configurational energy of the dislocations is accounted for explicitly, giving a self-consistent calculation of the conversion of plastic work to heat. The configurational energy is predicted to contribute to the mean pressure, and may reach several percent in the terapascal range, which may be significant when inferring scalar EOS data from dynamic loading experiments. The bulk elastic strain energy also contributes to the pressure, but appears to be much smaller. Although inherently describing the plastic relaxation of elastic strain, the model can be manipulated to estimate the flow stress as a function of mass density, temperature, and strain rate, which is convenient to compare with other models and inferences from experiment. The deduced flow stress reproduces systematic trends observed in elastic waves and instability growth experiments, and makes testable predictions of trends versus material and crystal type over a wide range of pressure and strain rate.

0 citations0 reviewsNo code linkedOpen access
preprint2022arXiv

Same and interconvertible high-pressure ice phases

Most experimentally known high-pressure ice phases have a body-centred cubic (bcc) oxygen lattice. Our atomistic simulations show that, amongst these bcc ice phases, ices VII, VII' and X are the same thermodynamic phase under different conditions, whereas superionic ice VII'' has a first-order phase boundary with ice VII'. Moreover, at about 300 GPa, ice X transforms into the Pbcm phase with a sharp structural change but no apparent activation barrier, whilst at higher pressures the barrier gradually increases. Our study thus clarifies the phase behaviour of the high-pressure insulating ices and reveals peculiar solid-solid transition mechanisms not known in other systems.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Atom-in-jellium equations of state and melt curves in the white dwarf regime

Atom-in-jellium calculations of the electron states, and perturbative calculations of the Einstein frequency, were used to construct equations of state (EOS) from around $10^{-5}$ to $10^7$g/cm$^3$ and $10^{-4}$ to $10^{6}$eV for elements relevant to white dwarf (WD) stars. This is the widest range reported for self-consistent electronic shell structure calculations. Elements of the same ratio of atomic weight to atomic number were predicted to asymptote to the same $T=0$ isotherm, suggesting that, contrary to recent studies of the crystallization of WDs, the amount of gravitational energy that could be released by separation of oxygen and carbon is small. A generalized Lindemann criterion based on the amplitude of the ion-thermal oscillations calculated using atom-in-jellium theory, previously used to extrapolate melt curves for metals, was found to reproduce previous thermodynamic studies of the melt curve of the one component plasma with a choice of vibration amplitude consistent with low pressure results. For elements for which low pressure melting satisfies the same amplitude criterion, such as Al, this melt model thus gives a likely estimate of the melt curve over the full range of normal electronic matter; for the other elements, it provides a useful constraint on the melt locus.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Atom-in-jellium predictions of the shear modulus at high pressure

Atom-in-jellium calculations of the Einstein frequency in condensed matter and of the equation of state were used to predict the variation of shear modulus from zero pressure to ~$10^7$ g/cm$^3$, for several elements relevant to white dwarf (WD) stars and other self-gravitating systems. This is by far the widest range reported electronic structure calculation of shear modulus, spanning from ambient through the one-component plasma to extreme relativistic conditions. The predictions were based on a relationship between Debye temperature and shear modulus which we assess to be accurate at the o(10%) level, and is the first known use of atom-in-jellium theory to calculate a shear modulus. We assessed the overall accuracy of the method by comparing with experimental measurements and more detailed electronic structure calculations at lower pressures.

0 citations0 reviewsNo code linkedOpen access
preprint2021arXiv

Equation of state and strength of diamond in high pressure ramp loading

Diamond is used extensively as a component in high energy density experiments, but existing equation of state (EOS) models do not capture its observed response to dynamic loading. In particular, in contrast with first principles theoretical EOS models, no solid-solid phase changes have been detected, and no general-purpose EOS models match the measured ambient isotherm. We have performed density functional theory (DFT) calculations of the diamond phase to ~10TPa, well beyond its predicted range of thermodynamic stability, and used these results as the basis of a Mie-Greuneisen EOS. We also performed DFT calculations of the elastic moduli, and calibrated an algebraic elasticity model for use in simulations. We then estimated the flow stress of diamond by comparison with the stress-density relation measured experimentally in ramp-loading experiments. The resulting constitutive model allows us to place a constraint on the Taylor-Quinney factor (the fraction of plastic work converted to heat) from the observation that diamond does not melt on ramp compression.

0 citations0 reviewsNo code linkedOpen access
preprint2020arXiv

Real-space formulation of the stress tensor for $\mathcal{O}(N)$ density functional theory: application to high temperature calculations

We present an accurate and efficient real-space formulation of the Hellmann-Feynman stress tensor for $\mathcal{O}(N)$ Kohn-Sham density functional theory (DFT). While applicable at any temperature, the formulation is most efficient at high temperature where the Fermi-Dirac distribution becomes smoother and density matrix becomes correspondingly more localized. We first rewrite the orbital-dependent stress tensor for real-space DFT in terms of the density matrix, thereby making it amenable to $\mathcal{O}(N)$ methods. We then describe its evaluation within the $\mathcal{O}(N)$ infinite-cell Clenshaw-Curtis Spectral Quadrature (SQ) method, a technique that is applicable to metallic as well as insulating systems, is highly parallelizable, becomes increasingly efficient with increasing temperature, and provides results corresponding to the infinite crystal without the need of Brillouin zone integration. We demonstrate systematic convergence of the resulting formulation with respect to SQ parameters to exact diagonalization results, and show convergence with respect to mesh size to established planewave results. We employ the new formulation to compute the viscosity of hydrogen at a million kelvin from Kohn-Sham quantum molecular dynamics, where we find agreement with previous more approximate orbital-free density functional methods.

0 citations0 reviewsNo code linkedOpen access
preprint2019arXiv

High temperature ion-thermal behavior from average-atom calculations

Atom-in-jellium calculations of the Einstein frequency were used to calculate the mean displacement of an ion over a wide range of compression and temperature. Expressed as a fraction of the Wigner-Seitz radius, the displacement is a measure of the asymptotic freedom of the ion at high temperature, and thus of the change in heat capacity from 6 to 3 quadratic degrees of freedom per atom. A functional form for free energy was proposed based on the Maxwell-Boltzmann distribution as a correction to the Debye free energy, with a single free parameter representing the effective density of potential modes to be saturated. This parameter was investigated using molecular dynamics simulations, and found to be ~0.2 per atom. In this way, the ion-thermal contribution can be calculated for a wide-range equation of state (EOS) without requiring a large number of molecular dynamics simulations. Example calculations were performed for carbon, including the sensitivity of key EOS loci to ionic freedom.

0 citations0 reviewsNo code linkedOpen access