The Crystal program for quantum-mechanical simulations of materials has been bridging the realm of molecular quantum chemistry to the realm of solid state physics for many years, since its first public version released back in 1988. This peculiarity stems from the use of atom-centered basis functions within a linear combination of atomic orbitals (LCAO) approach and from the corresponding efficiency in the evaluation of the exact Fock exchange series. In particular, this has led to the implementation of a rich variety of hybrid density functional approximations since 1998. Nowadays, it is acknowledged by a broad community of solid state chemists and physicists that the inclusion of a fraction of Fock exchange in the exchange-correlation potential of the density functional theory is key to a better description of many properties of materials (electronic, magnetic, mechanical, spintronic, lattice-dynamical, etc.). Here, the main developments made to the program in the last five years (i.e., since the previous release, Crystal17) are presented and some of their most noteworthy applications reviewed.

A general, versatile and automated computational algorithm to design any type of multiwall nanotubes of any chiralities is presented for the first time. It can be applied to rolling up surfaces obtained from cubic, hexagonal, and orthorhombic lattices. Full exploitation of the helical symmetry permits a drastic reduction of the computational cost and therefore opens to the study of realistic systems. As a test case, the structural, electronic, mechanical, and transport properties of multiwall carbon nanotubes (MWCNT) are calculated using a density functional theory approach, and results are compared with those of the corresponding layered (graphene-like) precursors. The interaction between layers has a general minimum for the inter-wall distance of ≈3.4 Å, in good agreement with experimental and computed optimal distances in graphene sheets. The metallic armchair and semiconductor zigzag MWCNT are almost isoenergetic and their stability increases as the number of walls increases. The vibrational fingerprint provides a reliable tool to identify the chirality and the thickness of the nanostructures. Finally, some promising thermoelectric features of the semiconductor MWCNT are reproduced and discussed.

In the framework of ab initio simulations, the search for energy minimum atomic structures is the first step to perform in studying the properties of a system. One of the most used and efficient optimization algorithms is a quasi-Newton line-search scheme based on the Broyden–Fletcher–Goldfarb–Shanno (Bfgs) Hessian updating formula. However, recent studies [Bitzek et al., Phys. Rev. Lett. 97, 170201 (2006) and Guénolé et al., Comput. Mater. Sci. 175, 109584 (2020)] suggested that minimization methods based on molecular dynamics concepts, such as the Fast Inertial Relaxation Engine (Fire) algorithm, often exhibit better performance and accuracy in finding local minima than line-search based schemes. In the present work, the implementation of Fire, in the framework of Crystal ab initio quantum mechanical simulation package [Dovesi et al., Wiley Interdiscip. Rev.: Comput. Mol. Sci. 8, e1360 (2018)], has been described. Its efficiency and performance in comparison with Bfgs quasi-Newton scheme have been assessed using Hartree–Fock and density functional theory with Perdew–Burke–Ernzerhof and hybrid functionals to model the potential energy surface. Fire shows good convergence behavior for all the considered systems, well reproducing the minimum energy structures obtained by the Bfgs approach. As regards the computational cost, Fire requires more iterations to converge with respect to Bfgs, but each Fire iteration is faster than the Bfgs one. The overall efficiency of Fire improves as the size of the system increased so that this minimization method seems to be very promising for systems without symmetry (space group P1) with a large number of atoms.

Interfacial Phase Change Memories (iPCM) based on (GeTe)2/Sb2Te3 superlattices have been proposed as an alternative candidate to conventional PCM for the realization of memory devices with superior switching properties. The...