2 research outputs found
Analytic Gradients for the Effective Fragment Molecular Orbital Method
The
analytic gradient for the Coulomb, polarization, exchange-repulsion,
and dispersion terms of the fully integrated effective fragment molecular
orbital (EFMO) method is derived and the implementation is discussed.
The derivation of the EFMO analytic gradient is more complicated than
that for the effective fragment potential (EFP) gradient, because
the geometry of each EFP fragment is flexible (not rigid) in the EFMO
approach. The accuracy of the gradient is demonstrated by comparing
the EFMO analytic gradient with the numeric gradient for several systems,
and by assessing the energy conservation during an EFMO NVE ensemble
molecular dynamics simulation of water molecules. In addition to facilitating
accurate EFMO geometry optimizations, this allows calculations with
flexible EFP fragments to be performed
Mechanisms Underlying Ion Transport in Lamellar Block Copolymer Membranes
Recent experiments have reported intriguing trends for
the molecular
weight (MW) dependence of the conductivity of block copolymer lamellae
that contrast with the behavior of homopolymer matrices. By using
coarse-grained simulations of the sorption and transport of penetrant
cations, we probe the possible mechanisms underlying such behavior.
Our results indicate that the MW dependence of conductivity of homopolymeric
and block copolymeric matrices arise from different mechanisms. On
the one hand, the solvation energies of cations, and, in turn, the
charge carrier concentrations, themselves, exhibit a MW dependence
in block copolymer matrices. Such trends are shown to arise from variations
in the thickness of the conducting phase relative to that of the interfacial
zones. Moreover, distinct mechanisms are shown to be responsible for
the diffusivities of ions in homopolymer and block copolymer matrices.
In the former, diffusivity effects associated with the free ends of
the polymers play an important role. In contrast, in block copolymer
lamellae, the interfacial zone between the blocks presents a zone
of hindered diffusivity for ions and manifests as a molecular weight
dependence of the ionic diffusivity. Together, the preceding mechanisms
are shown to provide a plausible explanation for the experimentally
observed trends for the conductivity of block copolymer matrices