Hybrid quantum mechanical/molecular mechanical (QM/MM) calculations have been carried out to investigate the structures of the neutral (H(2)O)(21) and protonated H(+)(H(2)O)(21) clusters confined in the crystal hosts. The influence of other cocrystallized species and the local electrostatic environments of the crystal hosts on the structures of water clusters has been analyzed. For the neutral (H(2)O)(21) duster in the tetrahedral host, its low-lying structures are found to exist as a dodecahedral cage with one interior water molecule, which is in good accord with the corresponding X-ray data. The confined (H(2)O)(21) cluster possesses the main structural features of the lowest-energy structure of the free (H(2)O)(21) cluster in the gas phase. For the protonated H(+)(H(2)O)(21) cluster confined in the cubic cavity, its low-lying structures are found to have a common hexahedral (H(2)O)(20) shell, which is consistent with the experimental X-ray structure, but the position of the additional H(2)O (or the H(3)O(+) ion) in different low-lying structures is different, while the H(3)O(+) ion is situated at the center of the cage in the corresponding X-ray structure. The overall shape of the confined H(+)(H(2)O)(21) cluster is significantly different from that of the free duster in the gas phase, and the confined cluster has much less intrinsic hydrogen bonds (H-bonds) than the free cluster, due to the need to form extrinsic H-bonds with acetonitrile molecules in the crystal host. The local electrostatic environment of the crystal host is found to exert a negligible influence on the structure of the (H(2)O)(21) cluster but play a significant role in modulating the structure of the H(+)(H(2)O)(21) cluster. This may be attributed to the fact that the protonated water cluster is much more easily polarized by the external electrostatic field of the crystal host than the neutral water cluster.