Aliphatic N-oxides as cosolvents with water play an important role in stabilizing and destabilizing the structure of biopolymers such as cellulose and proteins. To allow for detailed microscopic investigations, an empirical force field to be used in molecular simulations is developed for two N-oxide species, N,N,N-trimethylamine-N-oxide (TMAO) and N-methylmorpholine-N-oxide (NMMO). The intra-and intermolecular force field is parametrized mainly on the basis of quantum-chemical calculations and is tested against available experimental spectroscopic, crystallographic, and liquid state data. Special emphasis is put on the identification of transferable potential terms in order to guide future parametrization of other species. By construction, the force field is compatible with widely used potential functions for proteins and carbohydrates. With the resulting parameter set, molecular dynamics simulations are carried out on binary mixtures of water and N-oxides, revealing structural features and the influence of intramolecular N-oxide flexibility. Limitations and possible extensions of the presented models are also discussed.

Multidimensional potential energy surfaces for systems larger than about 15 atoms are so complex that interpreting their topographies and the consequent dynamics requires statistical analyses of their minima and saddles. Sequences of minimum-saddle-minimum points provide a characterization of such surfaces. Two examples, Ar 19 and (KCI) 32 , illustrate how topographies govern tendencies to form glasses or “focused” structures, for example, crystals or folded proteins. Master equations relate topographies to dynamics. The balance between glass-forming and structure-seeking characters of a potential energy surface seems governed by sawtooth versus staircase topography and the associated collectivity of the growth process after nucleation.

We explore the distance fluctuation criterion ͑''Lindemann criterion''͒ for melting transitions. Distances from average positions in accord with Lindemann, or interparticle distances, in accord with Jellinek and Berry or Etters and Kaelberer, are examined. The primary goal is to determine which of these offers the more useful criterion. The choice of origin can sometimes effect the significance of the index. We study three systems with two kinds of potentials. They are all composed of 64 particles: ͑a͒ and ͑b͒, a homopolymer and a cluster that consist of beads interacting pairwise through square-well potentials, and ͑c͒ a cluster of particles interacting pairwise through Morse potentials. For each of the noncrystalline structures, in contrast to the crystals originally studied by Lindemann, the fluctuation parameter based on interparticle distances gives a clearer separability of liquid and solid phases than that based on fluctuations from average positions. The solid-like forms of the Morse cluster, the square-well cluster, and the square-well homopolymer have similar behavior, indicating that a broad class of systems can be evaluated with this index. In these systems, relative fluctuation parameters provide a suitable criterion for the melting transition. The critical values for the interparticle distance criterion, which are in the range of 0.03-0.05, are smaller than those for the Lindemann criterion ͑0.1-0.15͒.

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