  By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio
calculations, we search for topological surface states and realizations of
Majorana bound states. Proposed topological states are expected to occur for
non-stoichiometric compositions on a surface Dirac cone where issues like
disorder scattering and charge transfer between relevant electronic states have
to be addressed. However, this surface Dirac cone is well above the
Fermi-level. Our goal is to theoretically design a substituted crystal in which
the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk
material without disturbing the surface. Going beyond conventional density
functional theory (DFT), we apply the coherent potential approximation
(BEB-CPA) in a mixed basis pseudo-potential framework to scan the
substitutional phase-space of co-substitutions on the Se-sites. We have
identified iodine as a promising candidate for intrinsic doping. Our specific
proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate
to exhibit a Dirac cone right at the Fermi energy without inducing strong
disorder scattering.
