  Using the coherent-potential approximation, we have studied the effects of
excess Fe, Se-deficiency, and substitutions of S, Te on Se sub-lattice and Co,
Ni and Cu on Fe sub-lattice in FeSe. Our results show that (i) a small amount
of excess Fe substantially disorders the Fe-derived bands while Se-deficiency
affects mainly the Se-derived bands, (ii) the substitution of S or Te enhances
the possibility of Fermi surface nesting, specially in FeSe$_{0.5}$Te$_{0.5}$,
in spite of disordering the Se-derived bands, (iii) the electron doping through
Co, Ni or Cu disorders the system and pushes down the Fe-derived bands, thereby
destroying the possibility of Fermi surface nesting. A comparison of these
results with the rigid-band, virtual-crystal and supercell approximations
reveals the importance of describing disorder with the coherent-potential
approximation.
