  With its array of 3840 Transition Edge Sensors (TESs) operated at 90 mK, the
X-Ray Integral Field Unit (X-IFU) on board the ESA L2 mission Athena will
provide spatially resolved high-resolution spectroscopy (2.5 eV FWHM up to 7
keV) over the 0.2 to 12 keV bandpass. The in-flight performance of the X-IFU
will be strongly affected by the calibration of the instrument. Uncertainties
in the knowledge of the overall system, from the filter transmission to the
energy scale, may introduce systematic errors in the data, which could
potentially compromise science objectives - notably those involving line
characterisation e.g. turbulence velocity measurements - if not properly
accounted for. Defining and validating calibration requirements is therefore of
paramount importance. In this paper, we put forward a simulation tool based on
the most up-to-date configurations of the various subsystems (e.g. filters,
detector absorbers) which allows us to estimate systematic errors related to
uncertainties in the instrumental response. Notably, the effect of
uncertainties in the energy resolution and of the instrumental quantum
efficiency on X-IFU observations is assessed, by taking as a test case the
measurements of the iron K complex in the hot gas surrounding clusters of
galaxies. In-flight and ground calibration of the energy resolution and the
quantum efficiency is also addressed. We demonstrate that provided an accurate
calibration of the instrument, such effects should be low in both cases with
respect to statistics during observations.

Instrumentation and Methods for Astrophysics