Highly efficient nature of the Kohn-Sham equations within density functional theory (DFT), and ever-increasing computer power has made electronic structure of complex systems easily accessible. DFT codes are routinely used for materials search and design, before performing actual expensive experiments. A reliable theory of groundstate properties is thus a key requirement for tailoring future materials and this in turn requires highly 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 linearized augmented-plane wave (FP-LAPW) code with many advanced features, and which has been in development now for 12 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 apart 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: magnetization 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 generalized gradient approximation (GGA) functional ever invented

4. calculation of phonon dispersions and electron-phonon coupling parameters

5. the only solid state code capable of calculating 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 propagation for extended systems.

8. the only solid state code capable of real-time dynamics of spins and charge under strong electro-magnetic field and

9. 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 DFT+U. This gives Elk the ability to study properties of materials which are inaccessible to other codes. These features have made Elk used by over nine hundred people around the world.