We present results on the X–ray properties of clusters and groups of galaxies , extracted from a large cosmological hydrodynamical simulation .
We used the Tree+SPH code GADGET to simulate a concordance \Lambda CDM cosmological model within a box of 192 h ^ { -1 } { Mpc } 
on a side , 480 ^ { 3 } dark matter particles and as many gas particles .
The simulation includes radiative cooling assuming zero metallicity , star formation and supernova feedback .
The very high dynamic range of the simulation allows us to cover a fairly large interval of cluster temperatures .
We compute X–ray observables of the intra–cluster medium ( ICM ) for simulated groups and clusters and analyze their statistical properties .
The simulated mass–temperature relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters .
Also , with the adopted choices of \Omega _ { m } = 0.3 and \sigma _ { 8 } = 0.8 for matter density and power spectrum normalization , respectively , the resulting X–ray temperature function agrees with the most recent observational determinations .
The luminosity–temperature relation also agrees with observations for clusters with T \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ > $ } } 2 keV .
At the scale of groups , T \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ < $ } } 1 keV , we find no change of slope in this relation .
The entropy in central cluster regions is higher than predicted by gravitational heating alone , the excess being almost the same for clusters and groups .
We also find that the simulated clusters appear to have suffered some overcooling .
We find f _ { * } \simeq 0.2 for the fraction of baryons in stars within clusters , thus about twice as large as the value observed .
Interestingly , temperature profiles of simulated clusters are found to steadily increase toward cluster centers .
They decrease in the outer regions , much like observational data do at r \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ > $ } } 0.2 % r _ { vir } , while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions .
Our results thus demonstrate the need for yet more efficient sources of energy feedback and/or the need to consider additional physical process which may be able to further suppress the gas density at the scale of poor clusters and groups , and , at the same time , to regulate the cooling of the ICM in central regions .