  The energy carried away by neutral particles in ultra high energy cosmic ray
showers can not be detected by fluorescence detectors. This energy is usually
referred to as the "invisible energy". Since every shower has a fraction of
invisible energy, the energy determined using the fluorescence technique is
always less than the primary energy and a correction needs to be applied. This
correction, usually referred to as the "missing energy", can only be estimated
using Monte Carlo simulations.
  In this article we study in detail the influence that discarding low energy
particles from the simulation has on the estimation of the missing and
invisible energies. We found that although the effect is not important for the
invisible energy,an important bias on the missing energy is introduced that can
reach 30% or more depending on the low energy cut value.
  We present a prescription on how to correct for this bias in AIRES
simulations and give a novel missing energy parametrization including results
for photons and for the QGSJET-II hadronic model. We also show that although
missing and invisible energies are closely related they are conceptually
different ideas if we consider the medium contribution to the shower energy.
