To further confirm this, we set a new system of an isolated H 2 O molecule interacting with the same laser field. From the result shown in Figure , we observe that the first OH bond oscillates with a period of 8.62 fs and the maximum bond length reaches 0.94 Å, no signs of bond breaking. The population dynamics on HOMO and LUMO of isolated H 2 O molecule also tells that the rarely electron populations in H 2 O's AB orbital, namely, this laser doesn't excite the electrons to the unoccupied orbitals of H 2 O, so that the isolated H 2 O molecule doesn't split. These results suggest that the OH bond breaking in Au 20 @H 2 O is attributed to the photon-induced intermolecular electronic transfer from the metal to the H 2 O. Next we examine the dependence of the dissociation rate on the laser intensity and frequency. The dissociation rate of H 2 O molecule is defined as the inverse of time required for the first OH bond length to reach 2.00 Å. As shown in Figure , initially, the splitting rate increases linearly as E 0 max increases from 1.80 V/Å to 2.20 V/Å. A maximum rate appears at 2.20 V/Å. When the field intensity is further increased, the rate doesn't change obviously. To get insight on this phenomenon, we plot the time-evolution of OH bond length at E 0 max = 2.80 V/Å in Figure , and find that both of two OH bonds are dissociated at this case. The intensive laser field can drive two OH bonds to break one after another though the reaction rate doesn't change obviously. As the light field increases from 1.80 V/Å to 2.80 V/Å, there is a transition from water splitting to water fragmenting, and the dissociation rate of OH bond even decreases.