However, the exact functional is unknown and approximations are needed (see Sec. The KS approach needs an exchange-correlation (XC) functional and the corresponding XC potential. They have many of the properties of the true electron wave functions and of particular importance is that one can fill up these KS orbitals as a function of their KS energy yielding the true electron density. (KS), the interacting many-body system of electrons is mapped onto a non-interacting system of quasiparticles, characterized by KS orbitals with a specific KS energy. Following the method outlined by Kohn and Sham 3 3. many current methods use density functional theory (DFT), 2 2. Pople, Ab Initio Molecular Orbital Theory ( Wiley-VCH, 1986). While earlier approaches focused on dealing with the electrons via wave functions, 1 1. There are many different methods of theoretically modeling the behavior of electrons and atoms in materials. References to relevant applications and some examples are also given. The various options, properties, and available approximations for the exchange-correlation functional, as well as the external libraries or programs that can be used with WIEN2k, are mentioned. After a brief presentation of the APW+lo method, we review the usage, capabilities, and features of WIEN2k (version 19) in detail. Many properties can be calculated, ranging from the basic ones, such as the electronic band structure or the optimized atomic structure, to more specialized ones such as the nuclear magnetic resonance shielding tensor or the electric polarization. The APW+lo method, which considers all electrons (core and valence) self-consistently in a full-potential treatment, is implemented very efficiently in WIEN2k, since various types of parallelization are available and many optimized numerical libraries can be used. The WIEN2k program is based on the augmented plane wave plus local orbitals (APW+lo) method to solve the Kohn–Sham equations of density functional theory.
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