  FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using
density functional theory. Our results show that for FeSe, LDA seems better
approximation in terms of magnitude of magnetic energy whereas GGA
overestimates it largely. On the other hand for FeTe, GGA is better
approximation that gives experimentally observed magnetic state. It has been
shown that the height of chalcogen atoms above Fe layers has significant effect
on band structure, electronic density of states (DOS) at Fermi level N(EF) and
Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner
criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the
value of N(EF) is large, the same is close to be satisfied. Force minimization
done for FeTexSe1-x using supercell approach shows that in disordered system Se
and Te do not share same site and have two distinct z coordinates. This has
small effect on magnetic energy but no significant difference in band structure
and DOS near EF when calculated using either relaxed or average value of z for
chalcogen atoms. Thus substitution of Se at Te site decreases average value of
chalcogen height above Fe layers which in turn affect the magnetism and
Fermiology in the system. By using coherent-potential approximation for
disordered system we found that height of chalcogen atoms above Fe layer rather
than chalcogen species or disorder in the anion planes, affect magnetism and
shape of Fermi surfaces (FS), thus significantly altering nesting conditions,
which govern antiferromagnetic spin fluctuations in the system.
