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 O-2 atmosphere. The electronic transport properties are calculated. It is found that the O-2 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-O-2 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.