DYNGA: a general purpose QM-MM-MD program. I. Application to water

We present a general purpose QM-MM-MD engine (DYNGA) designed to test alternative hybrid Hamiltonians geared towards the treatment of problems of interest in structural biology including the use of experimental data constraints. In this first presentation we use DYNGA to explore the behaviour of a traditional QM-MM approach in the treatment of the water—water interaction. We find the potential energy hypersurface for the water dimer computed with the HF 4–31G*/TIP3P hybrid Hamiltonian tends to be too flat. We also explore the effect of using traditional QM-MM techniques on proton wires and conclude there is a need for improvement, possibly addressed by using polarizable force fields.     

Nonlinear development of the dust acoustic instability in acollisional dusty plasma

We study the nonlinear behavior of the low-frequency dust acoustic instability in a collisional dusty plasma by means of particle simulations. The instability arises due to the streaming of plasma ions and neutrals relative to charged dust grains. According to linear theory, the presence of collisions between the plasma ions and a neutral gas background reduces the growth rate of the instability. Nonlinearly, however, the presence of drifting neutrals maintains the initial relative drift between plasma and dust ions until the unstable waves grow to large amplitude and collisions due to wave-particle interactions exceed the neutral collisions. As a result, stronger nonlinear effects, as manifested by enhanced fluctuations, larger amounts of plasma and dust heating, and a temporary reduction of the relative drift velocity, can occur in the presence of collisions     

UV-induced Coulomb crystallization of dust grains in high-pressuregas

A scheme for forming a Coulomb lattice of positively charged dust grains in a high-pressure (P>torr) gas is discussed theoretically. The grains are charged positively by photoemission in the presence of a flux of ultraviolet (UV) photons with energy larger than the work function of the grains, but lower than the ionization potential of the background gas. Methods for levitating and containing the dust using photophoretic forces (both UV and non-UV), gas drag, and electrostatic forces are considered     

Proceedings of the conference on organic chemistry at clusters and surfaces

The Proceedings of the Conference on Organic Chemistry at Clusters and Surfaces will comprise Volume 3, Number 3 (September 1992); Volume 3, Number 4 (December 1992); and Volume 4, Number 1 (March 1993).     

Simply Connected Minimal Surfaces in R3 Transverse to Horizontal Planes

We consider simply connected minimal surfaces in Euclidean space and we give a characterisation of the helicoid.     

Infinite-variate wide-sense Markov processes and functional analysis for bounded operator-forming vectors

Let p, q be arbitrary parameter sets, and let $\text{H}$ be a Hilbert space. We say that x = (xⁱ)_i?q, xⁱ ? $\text{H}$, is a bounded operator-forming vector (?$\text{H}$_F^q) if the Gram matrix 〈x, x〉 = [(xⁱ, x^j)]_i?q,j?q is the matrix of a bounded (necessarily ≥ 0) operator on $\text{l}{}_{\mathrm{q}}{}^2$, the Hilbert space of square-summable complex-valued functions on q. Let A be p × q, i.e., let A be a linear operator from $\text{l}{}_{\mathrm{q}}{}^2$ to $\text{l}{}_{\mathrm{p}}{}^2$. Then exists a linear operator ǎ from (the Banach space) $\text{H}$_F^q to $\text{H}$_F^p on $\text{D}$(A) = {x:x ? $\text{H}$_F^q, $\text{A〈x, x〉}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ is p × q bounded on $\text{l}{}_{\mathrm{q}}{}^2$} such that y = ǎx satisfies y^j?σ(x) = {space spanned by the xⁱ}, 〈y, x〉 = A〈x, x〉 and $\text{〈y, y〉 = A〈x, x〉}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(A〈x, x〉}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^1$. This is a generalization of our earlier [J. Multivariate Anal.4 (1974), 166–209; 6 (1976), 538–571] results for the case of a spectral measure concentrated on one point. We apply these tools to investigate q-variate wide-sense Markov processes.     

Glow discharge lamps as electron sources for electron impact excitation

It has been found possible to utilize the plasma in the discharge lamp of a conventional ultraviolet photoelectron spectrometer as an electron source f     

On duration and smoothing rules in a demisyllable-based isolated-word recognition system

In a recently proposed approach to isolated-word recognition, word reference templates are constructed from a universal set of demisyllable units by concatenating the appropriate demisyllables for each vocabulary item. A dynamic time warping (DTW) algorithm is used to align test and reference patterns optimally. Nevertheless some sort of syllable duration preadjustment is necessary because of the large potential difference in duration between isolated and in-context syllables. We have found that a simple rule that reduces the length of rhyme (final) demisyllables in nonword-final stressed syllables to approximately half their isolated-syllable duration provides recognition accuracy as high as that attained through use of complex, highly context-sensitive rules. In addition to its practical application, this result can be regarded as a further demonstration of the power of DTW. We have also investigated the requirements for parameter smoothing at demisyllable boundaries. We find that an optimal window duration for smoothing is about 60-90 ms, but that failure to smooth reduces recognition accuracy only about 2% in an 1109 word test set; that linear and parabolic smoothing are equally effective; and that it does not appear that recognition accuracy can be improved by smoothing in certain phonetic contexts only. Taken together, these results can be viewed as confirming the suitability of the demisyllable as the basic unit in recognition systems.