RESUMEN
We demonstrate that a recently proposed adaptive optimization algorithm for forward flux sampling simulations [E. E. Borrero and F. A. Escobedo, J. Chem. Phys. 129, 024115 (2008)] can be easily applied within the framework of transition interface sampling. This optimization algorithm systematically identifies the kinetic bottlenecks along the order parameter used to partition phase space via interfaces and improves the statistical accuracy of the reaction rate constant estimate. In different versions of the algorithm, the number or the placement of the interfaces (or both) are varied in order to allocate the numerical effort in a balanced way. The algorithm is demonstrated for a simple two-dimensional model and for the dipole flip transition of icelike structures inside carbon nanotubes. For these test systems, the optimization yielded an efficiency increase by a factor of 2-15.
RESUMEN
Inside narrow pores, for instance, realized as carbon nanotubes, water forms structures that strongly differ from the structure of bulk liquid water or ice. Here we compute vibrational spectra of such systems using molecular dynamics simulation combined with quantum mechanical perturbation theory. We focus on the spectroscopic response of single-file water chains in pores with subnanometer diameter, finding characteristic signatures of dangling and hydrogen-bonded hydrogen configurations occurring in this particular form of water. These features in the absorption spectra permit us to distinguish single-file water from the stacked-ring structures that form in wider pores. As previously observed in bulk liquid water, the vibrational frequency of the OH stretch of an HDO molecule in a system of D(2)O molecules is essentially determined by the electric field acting at the position of the hydrogen atom, providing a way to link the spectroscopic response to the local charge distribution of specific molecular arrangements.