Charge transfer in small hydrogen bonded clusters

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Abstract
High level ab initio and density functional calculations on clusters of water with acetate, methylammonium, and dimethylphosphate show that charge is transferred from the hydrogen bond acceptor to the hydrogen bond donor. The amounts of charge transferred are small, between 0.01 and 0.05 electron per hydrogen bond, but increase nearly linearly with the number of hydrogen bonds. The transfer of charge is not an artifact of the computation, since charge is also transferred in the limit of zero basis set superposition error. Calculations on a number of hydrogen bonded clusters show that the semiempirical AM1 and PM3 methods give excellent agreement with high level MP2 charge transfer effects, especially for AM1. Our results indicate the importance of charge transfer in hydrogen bond interactions.