Commissioning, Performance and Calibration of Crystals of the Electromagnetic Calorimeter of CMS

The operation and general performance of the CMS electromagnetic calorimeter at $\sqrt{s}=7$~TeV are described. The first LHC collision data have been used to finalize the commissioning of ECAL readout and trigger and to verify the readiness of ECAL for data taking. Di-photon states of $\pi^{0}$ and $\eta$ have also been used to verify and tune the energy scale. The quality of the offline data reconstruction, from crystal level quanties to clusters, has been investigated and improved using known physics processes. Decays of $\pi^{0}$ into two photons as well as the $\phi$ invariance of the average energy deposition at a given pseudorapidity are utilized to equalize the response of the individual channels. Based on an integrated luminosity of about 250~$\mathrm{nb}^{-1}$, an inter-calibration precision of 0.6\% has been achieved in the central barrel ECAL, in the pseudorapidty regions $-0.8< \eta < 0.8$.