We propose a new class of Hamiltonian models of liquid water based on resolution of the monomeric unit into three effective point charges. Interacting through central forces only, the three charges automatically assume the molecular structure. Two important effects are built into this model which have been neglected in similar attempts: intramolecular modes of vibration and the capacity for self-dissociation in the liquid phase. In addition, a large number of microsopic properties of water Can be expressed in very simple terms for this representation: The pressure and internal energy, second virial coefficient, high-frequency elastic moduli, and dielectric function are discussed in explicit terms. A convenient algorithm for computing low-order quantum corrections (proportional to h') to thermodynamic properties is given as well. To illustrate the general class of central-force models, we provide a concrete realization which has been determined by fitting phenomenological potentials to a nearly linear hydrogen bond of proper energy and dimer configuration. In order to elucidate the microscopic consequences of assuming central-force interactions in water, we have investigated the energy variation of small polymers (dimers and trimers) and the solvated proton near their minimum energy configurations. On a qualitative level, the results of these initial computations provide considerable encouragement for the view that water molecule interactions can be realistically approximated by linear combinations of central forces.
The simulation technique of molecular dynamics has been used to investigate a central-force model for liquid water at 22 °C and 1 g/cm3. In this model, the same atomic pair potentials are used both for intramolecular and intermolecular interactions. Atomic pair correlation functions and velocity autocorrelation functions have been computed. The results demonstrate that stable, nonlinear, vibrating molecules are present, and that they form a random hydrogen-bond network of the type usually attributed to water. These exploratory calculations suggest that small modifications in the central potentials VHH, VOH, and VOO would produce a more faithful representation of real water.
Abstruct-This paper presents a fiber-optic subscriber loop architecture employing multichannel wavelength-division multiplexing (WDM) techniques. The architecture, which we call the passive photonic loop (PPL), eliminates the need for outside-plant multiplexing and routing electronics while maintaining complete compatibility with broad-band ISDN. We describe an experimental demonstration of the PPL employing 32-channel WDM and a combination of DFB laser and LED transmitters. A family of alternative PPL implementations are identified. Tradeoffs between these alternatives are explored, and related hardware issues, such as wavelength alignment and WDM component temperature sensitivity, are discussed. We show an economic analysis comparing the PPL with the use of dedicated fiber between the central oflice and customer premises. The analysis predicts that the PPL can achieve a cost advantage for at least 80 to 90 percent of those subscribers served in today's network, given DFB-laser and dense-WDM component cost reductions of approximately one order of magnitude from today's unit-quantity prices.
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