@article{7481, abstract = {{The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn–VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe–Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO.}}, author = {{Riefer, Arthur and Weber, Nils and Mund, Johannes and Yakovlev, Dmitri R. and Bayer, Manfred and Schindlmayr, Arno and Meier, Cedrik and Schmidt, Wolf Gero}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{21}}, publisher = {{IOP Publishing}}, title = {{{Zn–VI quasiparticle gaps and optical spectra from many-body calculations}}}, doi = {{10.1088/1361-648x/aa6b2a}}, volume = {{29}}, year = {{2017}}, } @article{10030, abstract = {{The vibrational properties of stoichiometric LiNbO3 are analyzed within density-functional perturbation theory in order to obtain the complete phonon dispersion of the material. The phonon density of states of the ferroelectric (paraelectric) phase shows two (one) distinct band gaps separating the high-frequency (~800 cm−1) optical branches from the continuum of acoustic and lower optical phonon states. This result leads to specific heat capacites in close agreement with experimental measurements in the range 0–350 K and a Debye temperature of 574 K. The calculated zero-point renormalization of the electronic Kohn–Sham eigenvalues reveals a strong dependence on the phonon wave vectors, especially near Γ. Integrated over all phonon modes, our results indicate a vibrational correction of the electronic band gap of 0.41 eV at 0 K, which is in excellent agreement with the extrapolated temperature-dependent measurements.}}, author = {{Friedrich, Michael and Riefer, Arthur and Sanna, Simone and Schmidt, Wolf Gero and Schindlmayr, Arno}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{38}}, publisher = {{IOP Publishing}}, title = {{{Phonon dispersion and zero-point renormalization of LiNbO3 from density-functional perturbation theory}}}, doi = {{10.1088/0953-8984/27/38/385402}}, volume = {{27}}, year = {{2015}}, } @article{18542, abstract = {{We present recent advances in numerical implementations of hybrid functionals and the GW approximation within the full-potential linearized augmented-plane-wave (FLAPW) method. The former is an approximation for the exchange–correlation contribution to the total energy functional in density-functional theory, and the latter is an approximation for the electronic self-energy in the framework of many-body perturbation theory. All implementations employ the mixed product basis, which has evolved into a versatile basis for the products of wave functions, describing the incoming and outgoing states of an electron that is scattered by interacting with another electron. It can thus be used for representing the nonlocal potential in hybrid functionals as well as the screened interaction and related quantities in GW calculations. In particular, the six-dimensional space integrals of the Hamiltonian exchange matrix elements (and exchange self-energy) decompose into sums over vector–matrix–vector products, which can be evaluated easily. The correlation part of the GW self-energy, which contains a time or frequency dependence, is calculated on the imaginary frequency axis with a subsequent analytic continuation to the real axis or, alternatively, by a direct frequency convolution of the Green function G and the dynamically screened Coulomb interaction W along a contour integration path that avoids the poles of the Green function. Hybrid-functional and GW calculations are notoriously computationally expensive. We present a number of tricks that reduce the computational cost considerably, including the use of spatial and time-reversal symmetries, modifications of the mixed product basis with the aim to optimize it for the correlation self-energy and another modification that makes the Coulomb matrix sparse, analytic expansions of the interaction potentials around the point of divergence at k=0, and a nested density and density-matrix convergence scheme for hybrid-functional calculations. We show CPU timings for prototype semiconductors and illustrative results for GdN and ZnO. }}, author = {{Friedrich, Christoph and Betzinger, Markus and Schlipf, Martin and Blügel, Stefan and Schindlmayr, Arno}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{29}}, publisher = {{IOP Publishing}}, title = {{{Hybrid functionals and GW approximation in the FLAPW method}}}, doi = {{10.1088/0953-8984/24/29/293201}}, volume = {{24}}, year = {{2012}}, } @article{18624, abstract = {{We investigate the performance of the GW approximation by comparison to exact results for small model systems. The role of the chemical potentials in Dyson's equation as well as the consequences of numerical resonance broadening are examined, and we show how a proper treatment can improve computational implementations of many-body perturbation theory in general. Exchange-only and GW calculations are performed over a wide range of fractional band fillings and correlation strengths. We thus identify the physical situations where these schemes are applicable.}}, author = {{Pollehn, Thomas Joachim and Schindlmayr, Arno and Godby, Rex William}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{6}}, pages = {{1273--1283}}, publisher = {{IOP Publishing}}, title = {{{Assessment of the GW approximation using Hubbard chains}}}, doi = {{10.1088/0953-8984/10/6/011}}, volume = {{10}}, year = {{1998}}, }