  We present first-principles calculations of the coupling of quasiparticles to
spin fluctuations in iron selenide and discuss which types of superconducting
instabilities this coupling gives rise to. We find that strong
antiferromagnetic stripe-phase spin fluctuations lead to large coupling
constants for superconducting gaps with $s_\pm$-symmetry, but these coupling
constants are significantly reduced by other spin fluctuations with small wave
vectors. An accurate description of this competition and an inclusion of band
structure and Stoner parameter renormalization effects lead to a value of the
coupling constant for an $s_\pm$-symmetric gap which can produce a
superconducting transition temperature consistent with experimental
measurements.
