How do oxygen molecules move into silver contacts and change their electronic transport properties?

We present first principles simulations of the elongation process of the silver contact at the O2 atmosphere. The electronic transport properties are calculated. It is found that the O2 molecule can move into the silver contact during elongation and the corresponding mechanism is given. We demonstrate that there are two transmission channels around the Fermi level in an Ag-O2 contact system. The breaking process of an Ag-Ag bond is found to play an important role in determining the evolution of the system conductance during the elongation.