arXiv subjects

Fotios Kalkavouras

Publications and source records attributed to Fotios Kalkavouras.

8 recordsLinked to original sources

arXiv2026

Ab initio path integral Monte Carlo study of the 2D uniform electron liquid at finite temperatures

We present extensive \emph{ab initio} path integral Monte Carlo (PIMC) simulations of the two-dimensional uniform electron gas (2DEG), covering a broad range of density parameters $r_s=0.1,\dots,50$ and temperatures $\Theta=k_\textnormal{B}T/E_\textnormal{Fermi}=0.5,\dots,16$. This allows us to analyze various structural, linear density response and spectral properties. We find clear evidence of a \emph{roton-type} feature in the dynamic structure factor at strong coupling and intermediate wavenumbers. We also benchmark novel dielectric theory implementations for the 2DEG~[Kalkavouras \emph{et al.}~arXiv:2601.14989] for structural and spectral properties across the liquid phase diagram. The PIMC results can be used to benchmark existing theories and approximations, and guide the development of new methodologies.

cond-mat.quant-gas↗

arXiv2026

Kinetic energy from the cubic sum rule of the dynamic structure factor

The third frequency moment sum rule of the dynamic structure factor $S(\mathbf{q},\omega)$ is explored for the first time as an alternative estimator of the kinetic energy $K$ of quantum many-body systems. As a practical example, the uniform electron gas at warm dense matter conditions is considered. First, $K$ is extracted from quasi-exact \emph{ab initio} path integral Monte Carlo results for the imaginary-time density--density correlation function $F(\mathbf{q},\tau)$ and the expected excellent self-consistency with the thermodynamic differentiation route is confirmed. Second, $K$ is extracted from approximate dielectric formalism results for $S(\mathbf{q},\omega)$ and it is observed that common semi-classical approximations lead to a wave-number dependent $K$ with an incorrect short-wavelength limit. Our results are expected to be of broad interest for a great variety of applications, including time-dependent density functional theory, dielectric formalism schemes and warm dense matter models, as well as for the design of dedicated x-ray Thomson scattering experiments with the potential to provide model-free access to the full electronic equation of state.

physics.plasm-ph↗

arXiv2026

Dielectric formalism of the 2D uniform electron gas at finite temperatures

We present a comprehensive analysis of the two-dimensional uniform electron gas (2D-UEG or more commonly 2DEG) at finite temperature, spanning a broad range of densities / coupling strengths ($0.01\le{r}_s\le20$) and temperatures / degeneracy parameters ($0.01\le\Theta= k_B T/E_F \le 10$). Within the self-consistent dielectric formalism, we construct two-dimensional versions of the Singwi-Tosi-Land-Sj\"olander (STLS) and hypernetted-chain (HNC) approximation based schemes. We benchmark the accuracy of the STLS and the HNC schemes against new state-of-the-art path-integral Monte Carlo data. We also report structural and thermodynamic properties across the full $(r_s,\Theta)$ phase diagram domain studied, identify regimes in which these schemes remain quantitatively reliable, and provide an accurate parametrization of the exchange--correlation free energy of the finite-temperature 2DEG.

cond-mat.str-el↗

arXiv2025

Accelerated free energy estimation in ab initio path integral Monte Carlo simulations

We present a methodology for accelerating the estimation of the free energy from path integral Monte Carlo simulations by considering an intermediate artificial reference system where interactions are inexpensive to evaluate numerically. Using the spherically averaged Ewald interaction as this intermediate reference system for the uniform electron gas, the interaction contribution for the free energy was evaluated up to 18 times faster than the Ewald-only method. Furthermore, a $\xi$-extrapolation technique was tested and applied to alleviate the fermion sign problem and to resolve the sign for large particle numbers. Combining these two techniques enabled the evaluation of the free energy for a system of 1000 electrons, where both finite-size and statistical errors are below chemical accuracy. The general procedure can be applied to systems relevant for planetary and inertial confinement fusion modeling with low to moderate levels of quantum degeneracy.

physics.chem-ph↗

arXiv2024

Dynamic properties of the warm dense uniform electron gas with the qSTLS dielectric scheme

The recently derived Fourier--Matsubara expansion of imaginary--time correlation functions comprises an exact result of linear response theory for finite-temperature quantum many-body systems. In its density--density version, the expansion facilitates systematic comparisons between quasi-exact \emph{ab initio} path integral Monte Carlo simulations and approximate dielectric formalism schemes at the level of the imaginary--time (density--density) correlation functions and the dynamic Matsubara local field corrections. On this theoretical basis, the dynamic properties of the quantum version of the Singwi--Tosi--Land--Sj\"olander scheme are analyzed for the paramagnetic warm dense uniform electron gas. The marginal improvement compared to the semi-classical version of the Singwi--Tosi--Land--Sj\"olander scheme is attributed to the weak Matsubara order dependence of the approximate dynamic Matsubara local field correction. The evaluation of the ideal density response function at the non-interacting occupation numbers is identified to constitute a general deficiency of the dielectric formalism, which calls for a reformulation in future works.

physics.plasm-ph↗

arXiv2024

Dynamic exchange-correlation effects in the strongly coupled electron liquid

We present the first quasi-exact \textit{ab initio} path integral Monte Carlo (PIMC) results for the dynamic local field correction $\widetilde{G}(\mathbf{q},z_l;r_s,Θ)$ in the imaginary Matsubara frequency domain, focusing on the strongly coupled finite temperature uniform electron gas. These allow us to investigate the impact of dynamic exchange--correlation effects onto the static structure factor. Our results provide a straightforward explanation for previously reported spurious effects in the so-called \textit{static approximation} [Dornheim \textit{et al.}, \textit{Phys.~Rev.~Lett.}~\textbf{125}, 235001 (2020)], where the frequency-dependence of the local field correction is neglected. Our findings hint at the intriguing possibility of constructing an analytical four-parameter representation of $\widetilde{G}(\mathbf{q},z_l;r_s,Θ)$ valid for a substantial part of the phase diagram, which would constitute key input for thermal density functional theory simulations.

cond-mat.str-el↗

arXiv2024

Fourier-Matsubara series expansion for imaginary-time correlation functions

A Fourier-Matsubara series expansion is derived for imaginary-time correlation functions that constitutes the imaginary-time generalization of the infinite Matsubara series for equal-time correlation functions. The expansion is consistent with all known exact properties of imaginary-time correlation functions and opens up new avenues for the utilization of quantum Monte Carlo simulation data. Moreover, the expansion drastically simplifies the computation of imaginary-time density-density correlation functions with the finite temperature version of the self-consistent dielectric formalism. Its existence underscores the utility of imaginary-time as a complementary domain for many-body physics.

cond-mat.str-el↗

arXiv2024

Revisiting the Vashishta-Singwi dielectric scheme for the warm dense uniform electron fluid

The finite temperature version of the Vashishta-Singwi (VS) dielectric scheme for the paramagnetic warm dense uniform electron fluid is revisited correcting for an earlier thermodynamic derivative error. The VS scheme handles quantum mechanical effects at the level of the random phase approximation and treats correlations via the density expansion of a generalized Singwi-Tosi-Land-Sjölander (STLS) closure that inserts a parameter determined by enforcing the compressibility sum rule. Systematic comparison with quasi-exact results, based on quantum Monte Carlo simulations, reveals a structural superiority of the VS scheme towards strong coupling and a thermodynamic superiority of the STLS scheme courtesy of a favorable cancellation of errors. Guidelines are provided for the construction of dielectric schemes that are expected to be more accurate but computationally costly.

cond-mat.quant-gas↗