Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userEquation of state of fluid methane from first principles with machine learning potentials
85
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The predictive simulation of molecular liquids requires models that are not only accurate, but computationally efficient enough to handle the large systems and long time scales required for reliable prediction of macroscopic properties. We present a new approach to the systematic approximation of the first-principles potential energy surface (PES) of molecular liquids using the GAP (Gaussian Approximation Potential) framework. The approach allows us to create potentials at several different levels of accuracy in reproducing the true PES, which allows us to test the level of quantum chemistry that is necessary to accurately predict its macroscopic properties. We test the approach by building potentials for liquid methane (CH$_4$), which is difficult to model from first principles because its behavior is dominated by weak dispersion interactions with a significant many-body component. We find that an accurate, consistent prediction of its bulk density across a wide range of temperature and pressure requires not only many-body dispersion, but also quantum nuclear effects to be modeled accurately.
rate research
Read More
The Hugoniot curves for shock-compressed molybdenum with initial porosities of 1.0, 1.26, 1.83, and 2.31 are theoretically investigated. The method of calculations combines the first-principles treatment for zero- and finite-temperature electronic contribution and the mean-field-potential approach for the ion-thermal contribution to the total free energy. Our calculated results reproduce the Hugoniot properties of porous molybdenum quite well. At low porosity, in particular, the calculations show a complete agreement with the experimental measurements over the full range of data. For the two large porosity values of 1.83 and 2.31, our results are well in accord with the experimental data points up to the particle velocity of 3.5 km/s, and tend to overestimate the shock-wave velocity and Hugoniot pressure when further increasing the particle velocity. In addition, the temperature along the principal Hugoniot is also extensively investigated for porous molybdenum.
An iterative Monte Carlo inversion method for the calculation of particle pair potentials from given particle pair correlations is proposed in this paper. The new method, which is best referred to as Iterative Ornstein-Zernike Inversion, represents a generalization and an improvement of the established Iterative Boltzmann Inversion technique [Reith, P{u}tz & M{u}ller-Plathe, J. Comput. Chem. 24, 1624 (2003)]. Our modification of Iterative Boltzmann Inversion consists of replacing the potential of mean force as an approximant for the pair potential with another, generally more accurate approximant that is based on a trial bridge function in the Ornstein-Zernike integral equation formalism. As an input, the new method requires the particle pair correlations both in real space and in the Fourier conjugate wavenumber space. An accelerated iteration method is included in the discussion, by which the required number of iterations can be greatly reduced below that of the simple Picard iteration that underlies most common implementations of Iterative Boltzmann Inversion. Comprehensive tests with various pair potentials show that the new method generally surpasses the Iterative Boltzmann Inversion method in terms of reliability of the numerical solution for the particle pair potential.
We present and discuss a wide-range hydrogen equation of state model based on a consistent set of ab initio simulations including quantum protons and electrons. Both the process of constructing this model and its predictions are discussed in detail. The cornerstones of this work are the specification of simple physically motivated free energy models, a general multiparameter/multiderivative fitting method, and the use of the most accurate simulation methods to date. The resulting equation of state aims for a global range of validity ($T = 1-10^9 K$ and $V_m = 10^{-9}-1 m^3/mol$), as the models are specifically constructed to reproduce exact thermodynamic and mechanical limits. Our model is for the most part analytic or semianalytic and is thermodynamically consistent by construction; the problem of interpolating between distinctly different models -often a cause for thermodynamic inconsistencies and spurious discontinuities- is avoided entirely.
Liquid hydrocarbons are often modeled with fixed, symmetric, atom-centered charge distributions and Lennard-Jones interaction potentials that reproduce many properties of the bulk liquid. While useful for a wide variety of applications, such models cannot capture dielectric effects important in solvation, self-assembly, and reactivity. The dielectric constants of hydrocarbons, such as methane and ethane, physically arise from electronic polarization fluctuations induced by the fluctuating liquid environment. In this work, we present non-polarizable, fixed-charge models of methane and ethane that break the charge symmetry of the molecule to create fixed molecular dipoles, the fluctuations of which reproduce the experimental dielectric constant. These models can be considered a mean-field-like approximation that can be used to include dielectric effects in large-scale molecular simulations of polar and charged molecules in liquid methane and ethane. We further demonstrate that solvation of model ionic solutes and a water molecule in these fixed-dipole models improve upon dipole-free models.
Correlated many-fermion systems emerge in a broad range of phenomena in warm dense matter, plasmonics, and ultracold atoms. Quantum hydrodynamics (QHD) complements common first-principles methods for many-fermion systems and enables simulations at larger length and longer time scales. While the quantum Bohm potential is central to QHD, we illustrate its failure for strong perturbations. We extend QHD to this regime by utilizing the many-fermion quantum Bohm potential. This opens up the path to more accurate simulations in strongly perturbed warm dense matter, inhomogeneous quantum plasmas, and on nano-structure surfaces at scales unattainable with first-principles algorithms. The many-fermion quantum Bohm potential might also have important astrophysical applications in developing conformal-invariant cosmologies.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
