  The differential cross section for isolated-photon production in $pp$
collisions is measured at a centre-of-mass energy of 13 TeV with the ATLAS
detector at the LHC using an integrated luminosity of 36.1 fb$^{-1}$. The
differential cross section is presented as a function of the photon transverse
energy in different regions of photon pseudorapidity. The differential cross
section as a function of the absolute value of the photon pseudorapidity is
also presented in different regions of photon transverse energy.
Next-to-leading-order QCD calculations from JETPHOX and SHERPA as well as
next-to-next-to-leading-order QCD calculations from NNLOJET are compared with
the measurement, using several parameterisations of the proton parton
distribution functions. The predictions provide a good description of the data
within the experimental and theoretical uncertainties.
