The influence of various strains on crystal and electronic structures of
superconducting FeSe has been studied ab initio. We consider changes in the
Fermi surface nesting with a vector Q=(0.5,0.5)*(2\pi /a) as crucial for rising
superconductivity (SC) mediated by spin-fluctuations (SF). Our results indicate
that the c-axis strained FeSe exhibits the most imperfect nesting, which
enhances SF and, hence, also SC. In turn, the ab-plane compressive strain
slightly weakens this} nesting while the tensile strain destroys it completely.
These findings are consistent with reported earlier experimental dependencies
of superconducting transition temperatures on strain in FeSe thin films
