Density Functional Theory is a well established tool for predicting mechanical properties of bulk materials. Recent progress facilitated by modern efficient codes and increased computational power has opened doors to studying also extended systems. Examples of such are interfaces and grain boundaries, both being the key building blocks of advanced structural materials.
In this talk, we will report on first principles calculations of mechanical properties of complex structural models. The first example is a CrN/AlN multilayer . The full quantum mechanical calculations of directionally-resolved Young's modulus yielded comparable results with a simple continuum model of Grimsditch and Nizzoli, which were further confirmed by our experiments. The continuum model fed with first principles data of bulk AlN and CrN allows, in return, a fast estimation of elastic properties of superlattices with various architectures.
Encouraged by this successful story, we have applied the same methodology to a symmetrical Sigma5 grain boundary in Ni3Al . To our surprise, the simple method of Grimsditch and Nizzoli failed in this case, confirming the inevitable role the quantum mechanical calculation play in materials science. We will analyse the reasons for this failure and point out the conceptual differences between a superlattice and a grain boundary.
 M. Friak, D. Tytko, D. Holec, P.-P. Choi, P. Eisenlohr, D. Raabe, and J. Neugebauer, New J. Phys. 17, 093004 (2015).
 M. Grimsditch and F. Nizzoli, Phys. Rev. B Condens. Matter 33, 5891 (1986).
 M. Friak, M. Vsianska, D. Holec, and M. Sob, submitted.