arXiv subjects

Samuel Cajahuaringa

Publications and source records attributed to Samuel Cajahuaringa.

7 recordsLinked to original sources

arXiv2026

The Uhlenbeck-Ford model in two dimensions: Reference system for fluid-phase free-energy calculations

We investigate the Uhlenbeck-Ford (UF) model as a reference system for free-energy calculations in two-dimensional (2D) fluids. The 2D virial coefficients are computed exactly up to tenth order and combined with molecular simulation data to construct highly accurate numerical representations of the equation of state and the excess Helmholtz free energy. We then determine the phase diagram of the model in order to establish the thermodynamic stability limits of the fluid phase and thereby identify the range of applicability of the UF model as a fluid reference system. In the course of this analysis, we identify the solid, hexatic, and fluid phases, and show that the fluid remains the only thermodynamically stable phase, independent of density, for scaling parameters up to $p\lesssim 70$. Finally, we demonstrate the practical applicability of the 2D UF model as a reference system through thermodynamic integration calculations of the free energy of a two-dimensional Lennard-Jones fluid.

cond-mat.stat-mech↗

arXiv2026

Defects, Corrugation and Temperature Govern Rarefied-Air Drag on Graphene Coatings

In rarefied atmospheric environments, where continuum fluid dynamics breaks down, aerodynamic drag is governed by gas-surface momentum exchange, making surface structure and chemistry key design knobs. Using molecular dynamics simulations, we show that coating the $\alpha$-Al2O3(0001) surface with graphene markedly reduces the tangential momentum accommodation coefficient (TMAC) of N2, shifting scattering toward more specular reflection and thereby lowering drag; we further benchmark this response against graphite. The reduction strengthens up to 900 K. While structural defects can increase TMAC via defect-induced corrugation and local atomic and electronic rearrangements, graphene retains its performance at experimentally relevant defect densities.

cond-mat.mtrl-sci↗

arXiv2021

Nonequilibrium free-energy calculation of phase-boundaries using LAMMPS

We present a guide to compute the phase-boundaries of classical systems using a dynamic Clausius-Clapeyron integration (dCCI) method within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The advantage of the dCCI method is because it provides coexistence curves spanning a wide range of thermodynamic states using relatively short single non-equilibrium simulations. We describe the state-of-the-art of non-equilibrium free-energy methods that allow us to compute the Gibbs free-energy in a wide interval of pressure and/or temperature. We present the dCCI method in details, discuss its implementation in the LAMMPS package and make available source code, scripts, as well as auxiliary files. As an illustrative example, we determine the phase diagram of silicon in a range of pressures covering from 0 to 15 GPa and temperatures as low as 400 K up to the liquid phase, in order to obtain the phase boundaries and triple point between diamond, liquid and beta-Sn phases.

cond-mat.mtrl-sci↗

arXiv2019

Revisiting the fragile-to-strong crossover in metallic glass-forming liquids: application to Cu$_x$Zr$_x$Al$_{100-2x}$

The fragile-to-strong crossover seems to be a general feature of metallic glass-forming liquids. Here, we study the behavior of shear viscosity, diffusion coefficient and vibrational density of states for Cu$_\text{x}$Zr$_\text{x}$Al$_{\text{100-2x}}$ alloy through molecular dynamics simulations. The results reveal that the fragile-to-strong temperature (T$_\text{fs}$) and the glass transition temperature (T$_\text{g}$) increase as the aluminum content becomes larger. The inverse of the diffusion coefficient as a function of temperature exhibits a dynamical crossover in the vicinity of T$_\text{g}$, at a much lower temperature than that predicted by nearly all previous studies. At the temperature in which the dynamical crossover occurs determined by the inverse of the diffusion coefficient, we found an excess of vibrational states at low frequencies, resembling a pronounced peak in the reduced vibrational density of states characteristic of a strong liquid. Finally, the behavior of the shear viscosity as a function of reduced temperature (T$_\text{g}$/T) also shows that, besides the fragile-to-strong crossover nearby T$_\text{g}$, another dynamical crossover is present near the onset of the supercooled regime.

cond-mat.soft↗

arXiv2018

Nonequilibrium free energy methods applied to magnetic systems: the degenerate Ising model

In this paper, we review the physical concepts of the nonequilibrium techniques for the calculation of free energies applied to magnetic systems using Monte Carlo simulations of different nonequilibrium processes. The methodology allows the calculation of the free energy difference between two different system Hamiltonians, as well as the free energy dependence on temperature and magnetic field for a given Hamiltonian. As an illustration of the effectiveness of this approach, we apply the methodologies to determine the phase diagram of a simple microscopic model, the degenerate Ising model. Our results show very good agreement with those obtained from analytical (theoretical) methods.

cond-mat.stat-mech↗

arXiv2018

Stochastic sampling of the isothermal-isobaric ensemble: phase diagram of crystalline solids from molecular dynamics simulation

A methodology to sample the isothermal-isobaric ensemble using Langevin dynamics is proposed, which combines novel features of geometric integrators for the equations of motion. By employing the Trotter expansion, the methodology generates a robust, symmetric and accurate numerical algorithm. In order to show that the proposed method correctly samples the phase-space, simulations in the isotropic NPT ensemble were carried out for two analytical examples. Also this method permits to study a solid-solid phase transition, by conducting a fully flexible-cell molecular dynamics simulation. Additionally, we present an efficient method to determine the Gibbs free energy in a wide interval of pressure along an isothermal path, which allows us to determine the transition pressure in a driven by pressure solid-solid phase transition. Our calculations show that the methodology is highly suitable for the study of phase diagram of crystalline solids.

cond-mat.stat-mech↗

arXiv2013

Revisiting Dynamics Near a Liquid-Liquid Phase Transition in Si and Ga: The Fragile-to-Strong Transition

Using molecular dynamics simulations we analyze the dynamics of two atomic liquids that display a liquid-liquid phase transition (LLPT): Si described by the Stillinger-Weber potential and Ga as modeled by the modified embedded-atom model (MEAM). In particular, our objective is to investigate the extent to which the presence of a dip in the self-intermediate scattering function is a manifestation of an excess of vibrational states at low frequencies and may be associated with a fragile-to-strong transition (FTST) across the LLPT, as suggested recently. Our results do not lend support to these suggestions. Specifically, in the case of Ga we observe the appearance of an excess of vibrational states at low frequencies, even in the absence of the appearance of a dip in the self-intermediate scattering function across the LLPT. Furthermore, studying the behavior of the shear viscosities traversing the LLPTs we find that, despite the development of a dip in the self-intermediate scattering function for the case of Si and its absence in Ga, both substances are fragile in character above and below their respective LLPT temperatures.

cond-mat.stat-mech↗