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What is FlapwMBPT?

FlapwMBPT (Flapw is for Full-potential Linearized Augmented Plane Wave method, MBPT is for Many Body Perturbation Theory) is a computer code implementing many body diagrammatic approaches to study electronic structure and ground state properties of materials in their crystalline state.

Essential feature of the code consists in recently implemented self-consistent diagrammatic approaches which go beyond GW approximation [Phys. Rev. B 94, 155101 (2016)]. This feature (let us call it scGWΓ, with Γ being vertex function) can be considered as a new tool to study correlation effects in solids. scGWΓ is an ab-initio method and it is diagrammatic by construction. Thus, its accuracy can be systematically improved. scGWΓ uses totally controllable approximations and, as a result, can directly probe the strength of correlations in a given material. As an illustration of the results, obtained with scGWΓ, the accuracy of the calculated band gaps of a number of semiconductors/insulators is presented in the figure below:

error_in_band_gaps

 As it can be judged from the figure, a popular approach for the band gap calculation, G0W0, actually underestimates the gaps. Another popular approach, quasiparticle self-consistent GW (QSGW), as well as the fully self-consistent GW (scGW), overestimates the gaps. Both flavors of scGWΓ (schemes B and D) show considerable improvement in the accuracy of the calculated band gaps.

Another recent achievement of the scGWΓ is the improvement in the accuracy of the calculated band width (wb) of simple metals and the electron gas. For example, the figure below demonstrates the reduction in the mismatch (calculated wb versus experimental wb) for sodium metal:

sodium_band_width

A disadvantage of scGWΓ is the fact that the number of diagrams beyond GW which one can include is limited by practical considerations. This fact makes the approach applicable only for the materials with weak to modest strength of correlations. Where exactly the boundary between modest and strong correlations is has yet to be explored though.

Besides scGWΓ, code FlapwMBPT offers other approaches as well. They include Density Functional Theory (DFT) in its Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) which can be used to generate an initial approximation for the Green function in many body approaches, but it can be used on its own as well. Also available are self-consistent Hartree-Fock approximation (or Hybrid functionals) and self-consistent GW approximation (scGW) [Phys. Rev. B 85, 155129 (2012)]. The latter can be combined with linearized quasi-particle approximation to form our version of self-consistent quasiparticle GW method (LQSGW) [Comp.Phys.Commun. 219, 407 (2017)].