arXiv subjects

Phanish Suryanarayana

Publications and source records attributed to Phanish Suryanarayana.

At least 37 records · Page 2Linked to original sources

arXiv2024

Overcoming the chemical complexity bottleneck in on-the-fly machine learned molecular dynamics simulations

We develop a framework for on-the-fly machine learned force field molecular dynamics simulations based on the multipole featurization scheme that overcomes the bottleneck with the number of chemical elements. Considering bulk systems with up to 6 elements, we demonstrate that the number of density functional theory calls remains approximately independent of the number of chemical elements, in contrast to the increase in the smooth overlap of atomic positions scheme.

physics.comp-ph↗

arXiv2024

Spectral scheme for atomic structure calculations in density functional theory

We present a spectral scheme for atomic structure calculations in pseudopotential Kohn-Sham density functional theory. In particular, after applying an exponential transformation of the radial coordinates, we employ global polynomial interpolation on a Chebyshev grid, with derivative operators approximated using the Chebyshev differentiation matrix, and integrations using Clenshaw-Curtis quadrature. We demonstrate the accuracy and efficiency of the scheme through spin-polarized and unpolarized calculations for representative atoms, while considering local, semilocal, and hybrid exchange-correlation functionals. In particular, we find that $\mathcal{O}$(200) grid points are sufficient to achieve an accuracy of 1 microhartree in the eigenvalues for optimized norm conserving Vanderbilt pseudopotentials spanning the periodic table from atomic number $Z = 1$ to $83$.

physics.comp-ph↗

arXiv2024

Version 2.0.0 -- SPARC: Simulation Package for Ab-initio Real-space Calculations

SPARC is an accurate, efficient, and scalable real-space electronic structure code for performing ab initio Kohn-Sham density functional theory calculations. Version 2.0.0 of the software provides increased efficiency, and includes spin-orbit coupling, dispersion interactions, and advanced semilocal as well as hybrid exchange-correlation functionals, where it outperforms state-of-the-art planewave codes by an order of magnitude and more, with increasing advantages as the number of processors is increased. These new features further expand the range of physical applications amenable to first principles investigation.

physics.comp-ph↗

arXiv2024

Self-consistent convolutional density functional approximations: Application to adsorption at metal surfaces

The exchange-correlation (XC) functional in density functional theory is used to approximate multi-electron interactions. A plethora of different functionals is available, but nearly all are based on the hierarchy of inputs commonly referred to as "Jacob's ladder." This paper introduces an approach to construct XC functionals with inputs from convolutions of arbitrary kernels with the electron density, providing a route to move beyond Jacob's ladder. We derive the variational derivative of these functionals, showing consistency with the generalized gradient approximation (GGA), and provide equations for variational derivatives based on multipole features from convolutional kernels. A proof-of-concept functional, PBEq, which generalizes the PBE$α$ framework where $α$ is a spatially-resolved function of the monopole of the electron density, is presented and implemented. It allows a single functional to use different GGAs at different spatial points in a system, while obeying PBE constraints. Analysis of the results underlines the importance of error cancellation and the XC potential in data-driven functional design. After testing on small molecules, bulk metals, and surface catalysts, the results indicate that this approach is a promising route to simultaneously optimize multiple properties of interest.

cond-mat.mtrl-sci↗

arXiv2024

On-the-fly machine learned force fields for the study of warm dense matter: application to diffusion and viscosity of CH

We develop a framework for on-the-fly machine learned force field (MLFF) molecular dynamics (MD) simulations of warm dense matter (WDM). In particular, we employ an MLFF scheme based on the kernel method and Bayesian linear regression, with the training data generated from Kohn-Sham density functional theory (DFT) using the Gauss Spectral Quadrature method, within which we calculate energies, atomic forces, and stresses. We verify the accuracy of the formalism by comparing the predicted properties of warm dense carbon with recent Kohn-Sham DFT results in the literature. In so doing, we demonstrate that ab initio MD simulations of WDM can be accelerated by up to three orders of magnitude, while retaining ab initio accuracy. We apply this framework to calculate the diffusion coefficients and shear viscosity of CH at a density of 1 g/cm$^3$ and temperatures in the range of 75,000 to 750,000 K. We find that the self- and inter-diffusion coefficients as well as the viscosity obey a power law with temperature, and that the diffusion coefficient results suggest a weak coupling between C and H in CH. In addition, we find agreement within standard deviation with previous results for C and CH but disagreement for H, demonstrating the need for ab initio calculations as presented here.

physics.comp-ph↗

arXiv2023

Kohn-Sham accuracy from orbital-free density functional theory via $Δ$-machine learning

We present a $Δ$-machine learning model for obtaining Kohn-Sham accuracy from orbital-free density functional theory (DFT) calculations. In particular, we employ a machine learned force field (MLFF) scheme based on the kernel method to capture the difference between Kohn-Sham and orbital-free DFT energies/forces. We implement this model in the context of on-the-fly molecular dynamics simulations, and study its accuracy, performance, and sensitivity to parameters for representative systems. We find that the formalism not only improves the accuracy of Thomas-Fermi-von Weizs{ä}cker (TFW) orbital-free energies and forces by more than two orders of magnitude, but is also more accurate than MLFFs based solely on Kohn-Sham DFT, while being more efficient and less sensitive to model parameters. We apply the framework to study the structure of molten Al$_{0.88}$Si$_{0.12}$, the results suggesting no aggregation of Si atoms, in agreement with a previous Kohn-Sham study performed at an order of magnitude smaller length and time scales.

physics.chem-ph↗

arXiv2023

Strain engineering of Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants: An ab initio study

We study the influence of mechanical deformations on the Zeeman and Rashba effects in synthesized transition metal dichalcogenide (TMD) nanotubes and their Janus variants from first principles. In particular, we perform symmetry-adapted density functional theory simulations with spin-orbit coupling to determine the variation in the Zeeman and Rashba splittings with axial and torsional deformations. We find significant splitting in molybdenum and tungsten nanotubes, for which the Zeeman splitting decreases with increase in strain, going to zero for large enough tensile/shear strains, while the Rashba splitting coefficient increases linearly with shear strain, while being zero for all tensile strains, a consequence of the inversion symmetry remaining unbroken. In addition, the Zeeman splitting is relatively unaffected by nanotube diameter, whereas the Rashba coefficient decreases with increase in diameter. Overall, mechanical deformations represent a powerful tool for spintronics in TMD nanotubes as well as their Janus variants.

cond-mat.mtrl-sci↗

arXiv2023

Ab-initio investigation of finite size effects in rutile titania nanoparticles with semilocal and nonlocal density functionals

In this work, we employ hybrid and generalized gradient approximation (GGA) level density functional theory (DFT) calculations to investigate the convergence of surface properties and band structure of rutile titania (TiO$_2$) nanoparticles with particle size. The surface energies and band structures are calculated for cuboidal particles with minimum dimension ranging from 3.7 Å (24 atoms) to 10.3 Å (384 atoms) using a highly-parallel real-space DFT code to enable hybrid level DFT calculations of larger nanoparticles than are typically practical. We deconvolute the geometric and electronic finite size effects in surface energy, and evaluate the influence of defects on band structure and density of states (DOS). The electronic finite size effects in surface energy vanish when the minimum length scale of the nanoparticles becomes greater than 10 Å. We show that this length scale is consistent with a computationally efficient numerical analysis of the characteristic length scale of electronic interactions. The surface energy of nanoparticles having minimum dimension beyond this characteristic length can be approximated using slab calculations that account for the geometric defects. In contrast, the finite size effects on the band structure is highly dependent on the shape and size of these particles. The DOS for cuboidal particles and more realistic particles constructed using the Wulff algorithm reveal that defect states within the bandgap play a key role in determining the band structure of nanoparticles and the bandgap does not converge to the bulk limit for the particle sizes investigated.

cond-mat.mtrl-sci↗

arXiv2023

Assessing the source of error in the Thomas-Fermi-von Weizsäcker density functional

We investigate the source of error in the Thomas-Fermi-von Weizsäcker (TFW) density functional relative to Kohn-Sham density functional theory (DFT). In particular, through numerical studies on a range of materials, for a variety of crystal structures subject to strain and atomic displacements, we find that while the ground state electron density in TFW orbital-free DFT is close to the Kohn-Sham density, the corresponding energy deviates significantly from the Kohn-Sham value. We show that these differences are a consequence of the poor representation of the linear response within the TFW approximation for the electronic kinetic energy, confirming conjectures in the literature. In so doing, we find that the energy computed from a non-self-consistent Kohn-Sham calculation using the TFW electronic ground state density is in very good agreement with that obtained from the fully self-consistent Kohn-Sham solution.

cond-mat.mtrl-sci↗

arXiv2023

On the Effective Mass of Mechanical Lattices with Microstructure

We present a general formalism for the analysis of mechanical lattices with microstructure using the concept of effective mass. We first revisit a classical case of microstructure being modeled by a spring-interconnected mass-in-mass cell. The frequency-dependent effective mass of the cell is the sum of a static mass and of an added mass, in analogy to that of a swimmer in a fluid. The effective mass is derived using three different methods: momentum equivalence, action equivalence, and dynamic condensation. These methods are generalized to mechanical systems with arbitrary microstructure. As an application, we calculate the effective mass of a $1$D composite lattice with microstructure modeled by a chiral spring-interconnected mass-in-mass cell. A reduced (condensed) model of the full lattice is then obtained by lumping the microstructure into a single effective mass. A dynamic Bloch analysis is then performed using both the full and reduced lattice models, which give the same spectral results. In particular, the frequency bands follow from the full lattice model by solving a linear eigenvalue problem, or from the reduced lattice model by solving a smaller nonlinear eigenvalue problem. The range of frequencies of negative effective mass falls within the bandgaps of the lattice. Localized modes due to defects in the microstructure have frequencies within the bandgaps, inside the negative-mass range. Defects of the outer, or macro stiffness yield localized modes within each bandgap, but outside the negative-mass range. The proposed formalism can be applied to study the odd properties of coupled micro-macro systems, e.g., active matter.

physics.class-ph↗

arXiv2023

A note on transversal flexoelectricity in two-dimensional systems

In a recent letter [Phys. Rev. Lett. 127, 216801 (2021)], the authors introduced the effective flexoelectric coefficient $μ^\textrm{2D}$ for quantifying the flexoelectric effect in 2D systems, and reported a disagreement with the flexoelectric coefficient $μ_\text{T}$ introduced in Ref. [Phys. Rev. Mat. 5, L030801 (2021)], attributed to the neglect of the metric term $φ^\text{M}$ -- quadrupolar moment of the unperturbed charge density -- in the formulation of Ref. [Phys. Rev. Mat. 5, L030801 (2021)]. Here, we show that the model in Ref. [Phys. Rev. Mat. 5, L030801 (2021)] is correct and is in agreement with that in Ref. [Phys. Rev. Lett. 127, 216801 (2021)]. The discrepancies in the numerical values of the coefficients arise due to the difference in their definitions: $μ_\textrm{T}$ measures changes in bending-induced out-of-plane polarization, whereas $μ^\textrm{2D}$ measures changes in bending-induced voltage drop across the 2D system.

cond-mat.mtrl-sci↗

arXiv2023

Towards understanding flexoelectricity at the nanoscale

We review the authors' recent works on flexoelectricity at the nanoscale [arXiv:2010.01747, arXiv:2010.13899], while emphasizing the role of continuum mechanics in interpreting the electromechanical response of quantum mechanical systems under bending.

cond-mat.mtrl-sci↗

arXiv2023

On the calculation of phonons in real-space density functional theory

We present an accurate and efficient formulation for the calculation of phonons in real-space Kohn-Sham density functional theory. Specifically, employing a local exchange-correlation functional, norm-conserving pseudopotential in the Kleinman-Bylander representation, and local form for the electrostatics, we derive expressions for the dynamical matrix and associated Sternheimer equation that are particularly amenable to the real-space finite-difference method, within the framework of density functional perturbation theory. In particular, the formulation is applicable to insulating as well as metallic systems of any dimensionality, enabling the efficient and accurate treatment of semi-infinite and bulk systems alike, for both orthogonal and non-orthogonal cells. We also develop an implementation of the proposed formulation within the high-order finite-difference method. Through representative examples, we verify the accuracy of the computed phonon dispersion curves and density of states, demonstrating excellent agreement with established planewave results.

physics.chem-ph↗

arXiv2023

GPU acceleration of local and semilocal density functional calculations in the SPARC electronic structure code

We present a GPU-accelerated version of the real-space SPARC electronic structure code for performing Kohn-Sham density functional theory calculations within the local density and generalized gradient approximations. In particular, we develop a modular math kernel based implementation for NVIDIA architectures wherein the computationally expensive operations are carried out on the GPUs, with the remainder of the workload retained on the CPUs. Using representative bulk and slab examples, we show that GPUs enable speedups of up to 6x relative to CPU-only execution, bringing time to solution down to less than 30 seconds for a metallic system with over 14,000 electrons, and enabling significant reductions in computational resources required for a given wall time.

physics.comp-ph↗

arXiv2023

Properties of carbon up to 10 million kelvin from Kohn-Sham density functional theory molecular dynamics

Accurately modeling dense plasmas over wide ranging conditions of pressure and temperature is a grand challenge critically important to our understanding of stellar and planetary physics as well as inertial confinement fusion. In this work, we employ Kohn-Sham density functional theory (DFT) molecular dynamics (MD) to compute the properties of carbon at warm and hot dense matter conditions in the vicinity of the principal Hugoniot. In particular, we calculate the equation of state (EOS), Hugoniot, pair distribution functions, and diffusion coefficients for carbon at densities spanning 8 g/cm$^3$ to 16 g/cm$^3$ and temperatures ranging from 100 kK to 10 MK using the Spectral Quadrature (SQ) method. We find that the computed EOS and Hugoniot are in good agreement with path integral Monte Carlo results and the SESAME database. Additionally, we calculate the ion-ion structure factor and viscosity for selected points. All results presented are at the level of full Kohn-Sham DFT MD, free of empirical parameters, average-atom, and orbital-free approximations employed previously at such conditions.

physics.plasm-ph↗

arXiv2022

Version 2.0.0 -- M-SPARC: Matlab-Simulation Package for Ab-initio Real-space Calculations

M-SPARC is a Matlab code for performing ab initio Kohn-Sham Density Functional Theory simulations. Version 2.0.0 of the software further extends its capability to include relativistic effects, dispersion interactions, and advanced semilocal/nonlocal exchange-correlation functionals. These features significantly increase the fidelity of first principles calculations that can be performed using M-SPARC.

cond-mat.mtrl-sci↗

arXiv2022

Ab initio study on the electromechanical response of Janus transition metal dihalide nanotubes

We study the electronic response of Janus transition metal dihalide (TMH) nanotubes to mechanical deformations using Kohn-Sham density functional theory. Specifically, considering twelve armchair and zigzag Janus TMH nanotubes that are expected to be stable from the phonon analysis of flat monolayer counterparts, we first compute their equilibrium diameters and then determine the variation in bandgap and effective mass of charge carriers with the application of tensile and torsional deformations. We find that the nanotubes undergo a linear and quadratic decrease in bandgap with tensile and shear strain, respectively. In addition, there is a continual increase and decrease in the effective mass of electrons and holes, respectively. We show that for a given strain, the change in bandgap for the armchair nanotubes can be correlated with the transition metal's in-plane $d$ orbital's contribution to the projected density of states at the bottom of the conduction band.

cond-mat.mtrl-sci↗

arXiv2022

On the bending of rectangular atomic monolayers along different directions: an ab initio study

We study the bending of rectangular atomic monolayers along different directions from first principles. Specifically, choosing the phosphorene, GeS, TiS$_3$, and As$_2$S$_3$ monolayers as representative examples, we perform Kohn-Sham density functional theory calculations to determine the variation in transverse flexoelectric coefficient and bending modulus with the direction of bending. We find that while the flexoelectric coefficient is nearly isotropic, there is significant and complex anisotropy in bending modulus that also differs between the monolayers, with extremal values not necessarily occurring along the principal directions. In particular, the commonly adopted orthotropic continuum plate model with uniform thickness fails to describe the observed variations in bending modulus for GeS, TiS$_3$, and As$_2$S$_3$. We determine the direction-dependent effective thickness for use in such continuum models. We also show that the anisotropy in bending modulus is not associated with the rehybridization of atomic orbitals.

cond-mat.mtrl-sci↗