  In this paper we use state-of-the-art N-body hydrodynamic simulations of a
cosmological volume of side 100Mpc to produce many galaxy clusters
simultaneously in both the standard cold dark matter (SCDM) cosmology and a
cosmology with a positive cosmological constant (LCDM). We have performed
simulations of the same volume both with and without the effects of radiative
cooling, but in all cases neglect the effects of star formation and feedback.
With radiative cooling clusters are on average five times less luminous in
X-rays than the same cluster simulated without cooling. The importance of the
mass of the central galaxy in determining the X-ray luminosity is stressed.
