Jacopo Baima

Quantum‐mechanical condensed matter simulations with CRYSTAL

The latest release of the Crystal program for solid‐state quantum‐mechanical ab initio simulations is presented. The program adopts atom‐centered Gaussian‐type functions as a basis set, which makes it possible to perform all‐electron as well as pseudopotential calculations. Systems of any periodicity can be treated at the same level of accuracy (from 0D molecules, clusters and nanocrystals, to 1D polymers, helices, nanorods, and nanotubes, to 2D monolayers and slab models for surfaces, to actual 3D bulk crystals), without any artificial repetition along nonperiodic directions for 0–2D systems. Density functional theory calculations can be performed with a variety of functionals belonging to several classes: local‐density (LDA), generalized‐gradient (GGA), meta‐GGA, global hybrid, range‐separated hybrid, and self‐consistent system‐specific hybrid. In particular, hybrid functionals can be used at a modest computational cost, comparable to that of pure LDA and GGA formulations, because of the efficient implementation of exact nonlocal Fock exchange. Both translational and point‐symmetry features are fully exploited at all steps of the calculation, thus drastically reducing the corresponding computational cost. The various properties computed encompass electronic structure (including magnetic spin‐polarized open‐shell systems, electron density analysis), geometry (including full or constrained optimization, transition‐state search), vibrational properties (frequencies, infrared and Raman intensities, phonon density of states), thermal properties (quasi‐harmonic approximation), linear and nonlinear optical properties (static and dynamic [hyper]polarizabilities), strain properties (elasticity, piezoelectricity, photoelasticity), electron transport properties (Boltzmann, transport across nanojunctions), as well as X‐ray and inelastic neutron spectra. The program is distributed in serial, parallel, and massively parallel versions. In this paper, the original developments that have been devised and implemented in the last 4 years (since the distribution of the previous public version, Crystal14, occurred in December 2013) are described.

Raman spectroscopic features of the neutral vacancy in diamond from ab initio quantum-mechanical calculations.

Quantum-mechanical ab initio calculations are performed to elucidate the vibrational spectroscopic features of a common irradiation-induced defect in diamond, i.e. the neutral vacancy. Raman spectra are computed analytically through a Coupled-Perturbed-Hartree-Fock/Kohn-Sham approach as a function of both different defect spin states and defect concentration. The experimental Raman features of defective diamond located in the 400-1300 cm(-1) spectral range, i.e. below the first-order line of pristine diamond at 1332 cm(-1), are well reproduced, thus corroborating the picture according to which, at low damage densities, this spectral region is mostly affected by non-graphitic sp(3) defects. No peaks above 1332 cm(-1) are found, thus ruling out previous tentative assignments of different spectral features (at 1450 and 1490 cm(-1)) to the neutral vacancy. The perturbation introduced by the vacancy to the thermal nuclear motion of carbon atoms in the defective lattice is discussed in terms of atomic anisotropic displacement parameters (ADPs), computed from converged lattice dynamics calculations.

Large Scale Condensed Matter DFT Simulations: Performance and Capabilities of the CRYSTAL Code

Nowadays, the efficient exploitation of high-performance computing resources is crucial to extend the applicability of first-principles theoretical methods to the description of large, progressively more realistic molecular and condensed matter systems. This can be achieved only by devising effective parallelization strategies for the most time-consuming steps of a calculation, which requires some effort given the usual complexity of quantum-mechanical algorithms, particularly so if parallelization is to be extended to all properties and not just to the basic functionalities of the code. In this Article, the performance and capabilities of the massively parallel version of the Crystal17 package for first-principles calculations on solids are discussed. In particular, we present: (i) recent developments allowing for a further improvement of the code scalability (up to 32 768 cores); (ii) a quantitative analysis of the scaling and memory requirements of the code when running calculations with several thousands (up to about 14 000) of atoms per cell; (iii) a documentation of the high numerical size consistency of the code; and (iv) an overview of recent ab initio studies of several physical properties (structural, energetic, electronic, vibrational, spectroscopic, thermodynamic, elastic, piezoelectric, topological) of large systems investigated with the code.

Electron density analysis of large (molecular and periodic) systems: A parallel implementation

A parallel implementation is presented of a series of algorithms for the evaluation of several one‐electron properties of large molecular and periodic (of any dimensionality) systems. The electron charge and momentum densities of the system, the electrostatic potential, X‐ray structure factors, directional Compton profiles can be effectively evaluated at low computational cost along with a full topological analysis of the electron charge density (ECD) of the system according to Baders quantum theory of atoms in molecules. The speedup of the parallelization of the different algorithms is presented. The search of all symmetry‐irreducible critical points of the ECD of the crystallized crambin protein and the evaluation of all the corresponding bond paths, for instance, would require about 32 days if run in serial mode and reduces to less than 2 days when run in parallel mode over 32 processors.

Direct Piezoelectric Tensor of 3D Periodic Systems through a Coupled Perturbed Hartree–Fock/Kohn–Sham Method

Abstract A quasi-analytical theoretical method is devised, and implemented in the Crystal program, for calculation of the direct “proper” piezoelectric tensor of periodic systems including both the clamped-nuclei electronic and nuclear relaxation contributions. It is based on using the analytical Coupled-Perturbed-Hartree–Fock/Kohn–Sham (CPHF/KS) procedure to obtain dipole derivatives with respect to lattice deformations as well as internal coordinates. The sole numerical step required involves building the Hessian matrix through differentiation of analytical energy gradients. Two prototypical piezoelectric, non-ferroelectric, crystals, namely ZnO and α-quartz, are used to demonstrate the accuracy and computational efficiency of our new scheme, which significantly improves upon the commonly used numerical Berry phase approach.

Spin susceptibility and electron-phonon coupling of two-dimensional materials by range-separated hybrid density functionals: Case study ofLixZrNCl

We investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility,

Beryllium Oxide Nanotubes and their Connection to the Flat Monolayer

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Thermodynamics and phonon dispersion of pyrope and grossular silicate garnets from ab initio simulations

, and the intervalley electron-phonon coupling in Li

Ab Initio Periodic Simulation of the Spectroscopic and Optical Properties of Novel Porous Graphene Phases

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The electronic states of the neutral vacancy in diamond: a quantum mechanical approach

ZrNCl. At low doping, Li