Properties of complex systems have become easily accessible thanks to the highly effecient nature of the Kohn-Sham equations within density functional theory (DFT), and also to ever-increasing computer power. DFT codes are now used for materials design, saving the time and expense of performing actual experiments. A reliable theory of groundstate properties is thus a key requirement for tailoring future materials and this in turn requires higly accurate numerical codes. There are several computer codes capable of calculating groundstate or excited state properties, but only a few of them are of the highly accurate all-electron kind, which treat all the electrons in the solid on the same footing. Codes which use an all-electron basis, like augmented plane waves (APW),
are the most accurate in use today and although more complicated for development, users can be assured of precise results, free from anomalies arising from the use of approximations like pseudopotentials.
THE ELK CODE
Elk is an all-electron full-potential linearised augmented-plane wave (FP-LAPW) code with many advanced features, and which has been in development now for ten years. It is released under the GNU General Public License (GPL), so as to encourage scrutiny of the code itself, free development of new techniques as well as a lively community of users. What sets Elk code appart from its contemporaries is
1. all-electron APW for highest accuracy calculations: this includes a flexible basis, which in addition to APW also includes local-orbitals of different kinds
2. highly general treatment of spins: magnetisation is treated as a free vector field allowing for non-collinear magnetic systems including spin-spiral states
3. interface to the ETSF exchange-correlation library libxc making available
almost every local density approximation (LDA) and generalised gradient approximation (GGA) functional ever invented
4. calculation of phonon dispersions and electron-phonon coupling parameters
5. the only solid state code capable of calculting ground state properties and spectra using one body reduced density matrix functional theory (RDMFT)
6. linear and non-linear optical spectrascopy with in random phase approximation; sophisticated calculation of linear optical spectra by solving linear response time-dependent density functional theory (TDDFT) equation using various exchange-correlation kernels as well as by solving the Bethe-Salpeter equation (BSE)
7. the only code capable of real time propogation for extended systems.
8. and most importantly in addition to being user friendly, it is highly developer friendly -- new ideas within the field of DFT can easily be implemented within Elk
What is unique to Elk is that it is specifically programmed so that most features can be used in combination with one another. For example, it is the only code that can produce a phonon spectrum for a non-collinear magnetic system in conjunction with LDA+U. This gives Elk the ability to study properties of materials which are inaccessible to other
These features have made Elk used by over six hundred people around the world. This number has rapidly increased in last two year, thanks in part to the previous ELK-CECAM workshop held in 2011. With steadily increasing use and development of the code, we propose that a CECAM tutorial be organized so that the main authors of the code and experts in the field of DFT can train new users and developers in both the fundamental physics as well as the practical aspects of running and development.