CRYSTAL and EMBED, two computational tools for the ab-initio study of the electronic properties of crystals.

The present study discusses the main features of the two programs CRYSTAL and EMBED developed for the ab initio study of the electronic properties of perfect periodic structures and of crystals with local defects, respectively. After a brief historical introduction, the structure of CRYSTAL is outlined and some specific aspects are discussed in detail: the use of local basis functions, the way of dealing with the Coulomb and the exchange series, the exploitation of point symmetry, the possibility to adopt either the Hartree–Fock approach or one among a variety of Kohn–Sham Hamiltonians. The present capabilities of the program are illustrated by a survey of selected applications from existing literature. Information is provided concerning work in progress aimed at removing some of the limitations of the code and improving its performance. The characteristics of EMBED are analyzed with emphasis given to the critical aspects of the method: limits of validity of the fundamental approximation on which the embedding technique relies, problems of convergence of the self-consistent procedure; and the delicate issue of the estimate of the defect formation energy. Again, a critical survey of applications clarifies the capabilities of the code. The envisaged improvements to be introduced in a forthcoming release of EMBED are presented. The present and prospective role of the two programs in the field of computational studies of condensed matter problems is outlined.