Simulations are carried out for the ice/vapor and ice/liquid interfaces using models for water which include intermolecular charge transfer, as well as polarizability. The models transfer a small amount of charge for each hydrogen bond formed, as indicated from electronic structure calculations, from the molecule that accepts the hydrogen bond to the molecule that donates the hydrogen bond. Depending on distance from the interface, molecules can, on average, have more of one type (donor or acceptor) than the other. The asymmetric local environment leads to net charge transfer at the interface, with layers of molecules with small net charges. Molecules at the ice side of the interface tend to be positively charged, while molecules at the vapor or liquid side tend to be negatively charged.
Replica exchange is a powerful simulation method in which simulations are run at a series of temperatures, with the highest temperature chosen so phase space can be sampled efficiently. In order for swaps to be accepted, the energy distributions of adjacent replicas must have some overlap. This can create the need for many replicas for large systems. In this paper, we present a new method in which the potential energy is scaled by a parameter, which has an explicit time dependence. Scaling the potential energy broadens the distribution of energy and reduces the number of replicas necessary to span a given temperature range. We demonstrate that if the system is driven by the time-dependent potential sufficiently slowly, then equilibrium is maintained and energetic and structural properties are identical to those of conventional replica exchange. The method is tested using two systems, the alanine dipeptide and the trpzip2 polypeptide, both in water.
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