Isotope quantum effects in water around the freezing point

We have measured the difference in electronic structure factors between liquid H(2)O and D(2)O at temperatures of 268 and 273 K with high energy x-ray diffraction. These are compared to our previously published data measured from 279 to 318 K. We find that the total structural isotope effect increases by a factor of 3.5 over the entire range, as the temperature is decreased. Structural isochoric temperature differential and isothermal density differential functions have been used to compare these data to a thermodynamic model based upon a simple offset in the state function. The model works well in describing the magnitude of the structural differences above approximately 310 K, but fails at lower temperatures. The experimental results are discussed in light of several quantum molecular dynamics simulations and are in good qualitative agreement with recent temperature dependent, rotationally quantized rigid molecule simulations.

Temperature dependence of isotopic quantum effects in water

The technique of high energy x-ray diffraction has been used to measure the temperature variation of hydrogen versus deuterium isotopic quantum effects on the structure of water. The magnitude of the effect is found to be inversely proportional to the temperature, varying by a factor of 2.5 over the range 6 to 45 degrees C. In addition, the H216O versus H218O effect has been measured at 26 degrees C and the structural difference shown to be restricted to the nearest neighbor molecular interactions. The results are compared to recent simulations and previously measured isochoric temperature differentials; additionally, implications for H/D substitution experiments are considered.

Structural studies of several distinct metastable forms of amorphous ice

Structural changes during annealing of high-density amorphous ice were studied with both neutron and x-ray diffraction. The first diffraction peak was followed from the high- to the low-density amorphous form. Changes were observed to occur through a series of intermediate forms that appear to be metastable at each anneal temperature. Five distinct amorphous forms were studied with neutron scattering, and many more forms may be possible. Radial distribution functions indicate that the structure evolves systematically between 4 and 8 angstroms. The phase transformations in low-temperature liquid water may be much more complex than currently understood.

Finite-size effects in molecular dynamics simulations: intermediate scattering function and velocity of sound. III. Theory and application to a model krypton fluid

We describe a method for obtaining the intermediate scattering function I(Q,t) from a computer simulation: it is an extension of our earlier calculation [Salacuse, Denton, and Egelstaff, Phys. Rev. E 53, 2382 (1996)] for the t-->0 limit. We use this approach to obtain I(Q,t) for low Q and t from molecular dynamics (MD) simulations of a model krypton fluid whose atoms interact via a truncated Aziz pair potential, and the results are compared over their range of validity to I(Q,t) determined by the standard MD method and also by a time expansion approach. In its range of validity our approach is much more efficient than the standard MD method; however, it covers a restricted range of t due to the movement of density fluctuations (sound waves) through the simulated fluid which produces an anomaly in the time behavior of I(Q,t). By analyzing I(Q=0,t) the velocity of sound in the simulation is determined, and the results compare favorably with published experimental results for the sound velocity of liquid krypton.

Experimental tests for many-body forces in noble gas fluids

Effects due to many-body potentials may be studied by comparing the predictions of precise computer simulations (using a good pair potential) with real experimental data. The differences observed may be of almost any relative magnitude, varying with state parameters and other experimental variables. Results of this kind will be discussed regarding P.V.T. data, transport coefficients, and static and dynamic structure factors of noble gas fluids. They are compared to the predictions of a simple mean field model for each effect, in which pair potential simulation is used as the reference system. The model is used as a tool to determine the information content of each property. Reasonable agreement is found for bulk properties, while the extra sensitivity offered by the microscopic data allows a number of deficiencies in the model to be observed. In contrast, a similar series of tests using a computer model, made from the pair potential and the A–T potential in place of the experimental data, shows significantly better overall agreement with the mean field model. This is believed to be due to the neglect of important short range terms in the A–T model. It is concluded that at least three types of many-body forces are important in discussing dense noble gas fluids.

Inelastic scattering of neutrons and possible biological applications

The field of neutron inelastic scattering has probably been developed to the stage where it can begin to help the biologist. Because essentially no experimental data have been obtained, it is difficult either to draw conclusions or to make forecasts except on the basis of general hypotheses. It seems likely, however, that the next stage is up to biologists. After reviewing those biological problems in which molecular dynamics might play an important role, they should suggest specimens of interest which can give inelastic peaks with existing spectrometers operating with 5 to 10-A neutrons at angles greater than 5degrees and with resolutions of approximately 50 mueV. These specimens may involve molecules slightly smaller and more mobile than some biologists would like, but a successful outcome might lead to the development of spectrometers capable of working in a more satisfactory range. In this event the return may well prove rewarding to the biologists.

British High flux Beam Reactor

The neutron scattering technique has become an accepted method for the study of condensed matter. Because of the great scientific and technical value of neutron experiments and the growing body of users, several proposals have been made during the past decade for a nuclear reactor devoted primarily to this technique. This article reviews the reasons for and history behind these proposals.