The Back Ends

The back end is the component of HyperChem that performs the more time-consuming scientific calculations. This is where molecular mechanical and quantum mechanical calculations are performed. The back end can be thought of as the computational chemistry component of HyperChem. 

Aback end program essentially acts as a computational server for the front end. It receives input, computes something, sends it back to the front end and then looks for further input or commands from the front end. It can be used over and over for different calculations. Hit has been explicitly stopped, the front end will start 

The internal architecture of HyperChem back ends is different from that expected to be used by third-party packages. To a third-party agent wishing to interface with HyperChem, HyperChem always acts as a server. Thus a third-party molecular dynamics package would ask HyperChem to send the coordinates of a molecule rather than HyperChem determining on its own that it should send coordinates at the appropriate time. 

The HyperChem philosophy associated with back end computations is one which is intended to instill confidence, as far as is possible, in the scientific results emanating from HyperChem. This philosophy is one of openness — openness about the product, the calculations being performed, the science embodied in the product, etc. Apart from protecting the proprietary code associated with a commercial product. Hypercube wishes to document and describe as fully as is possible the calculations that HyperChem performs. There should be no mystery about the scientific results obtained with HyperChem. 

HyperChem should not be viewed as a black box that computes only what its designers thought correct. It has an open architecture that makes it possible to customize it many ways. As far as is possible, the parameters of molecular mechanics and semi-empirical calculations are in the user s hands. As the techniques of software engineering advance and our expertise in building new 

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Diagram showing a flow of ions of m/z a, b, c, etc. traveling in bunches toward the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves toward the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ion.s of m/z a, b, c, etc. are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to V m/z). 

For a microchannel plate, the back end of each element is left open, as shown in Figures 30.5 or 30.6, and forms a microchannel. Any electrons emerging from any element are all detected by the one collector plate. 

Development of molybdenum electrodes in the 1950s permitted the use of electrically assisted melting in regenerative furnaces (81). In the 1990s, approximately one-half of all regenerative tanks ate electrically boosted. Operating practice has shown that effective use of electricity near the back end of the furnace, where the batch is added, can reduce fossil fuel needs. This lowers surface temperature and reduces batch volatilisation. 

In the soldedess wrap (Fig. 3) or wire-wrap connection, a wire conductor is coiled around the back end of the separable contact, which has a square or rectangular cross section (4). The corners of the soldedess wrap post and the areas of the wire that are in contact with it are severely deformed. In a propedy made wrap, the force required to sHde the wire along the post exceeds the breaking strength of the wire. The method is suitable only for soHd wire, and special tools are used to make this connection. 

Slotted beam or U-contacts describe a versatile design for the termination of soHd wire and require that the wire be pushed into a narrow slot between two moderately rigid tines, or beams, at the back end of the separable contact (Fig. 8). The edges of the beams displace the insulation, squee2e the wire, and keep it in compression for the life of the connection. This termination method was developed for terminating conductors in a gang using flexible flat cables with round conductors (5). 

Methods used to secure a wire to the back end of a separable contact include the taper pin and the soldedess cHp. The former is a cylindrical tapered body having a hoUow end into which a wire is crimped the front of the taper pin is forced into the back end of the connector contact which has conforming shape. The soldedess cHp has a spring which traps the soHd or stranded wire against a post at the back end of the separable contact the cHp encircles both the wire and the post (6). 

The scratch marks on the circumference of these close tolerance rotary parts will correspond to. scratch marks on close tolerance stationary parts at approximately 60 around the volute from the cutwater. These marks will be vi.sible on the back plate with open impellers, or on the wear rings of pumps with enclosed impellers, or the ID bore of the restriction bushing at the bottom of the seal chamber where the shaft pa.sses through, or the ID of the seal chamber bore at the back end of the mechanical seal (Figure 9-6 and Figure 9-7, next page). 

Instead of using conventional refrigeration and condensers to convert incoming gas into LNG, the system was designed to feed 2,340 Nm / day (86,800 scfd) into each of two expanders. Both are connected in parallel, with compressors attached at the back end. The compressors are connected in series to boost a single stream of returning gas. 

The Btu needed to evaporate this quantity of water and to raise the vapor to the back-end temperature of the kiln is ... 

Petroleum engineers are traditionally involved in activities known in the oil industry as the front end of the petroleum fuel cycle (petroleum is either liquid or gaseous hydrocarbons derived from natural deposits—reservoirs—in the earth). These front end activities are namely exploration (locating and proving out the new geological provinces with petroleum reservoirs that may be exploited in the future), and development (the systematic drilling, well completion, and production of economically producible reservoirs). Once the raw petroleum fluids (e.g., crude oil and natural gas) have been produced from the earth, the back end of the fuel cycle takes the produced raw petroleum fluids and refines the.se fluids into useful products. 

Tailpipe emissions of HC and CO are affected by the levels of heavy aromatics in gasoline. Like sulfur, the heavy aromatics are in the back end of the boiling range (Figure 10-4). As with sulfur, reduction of end point directly controls the concentration of heavy aromatics in finished gasoline. 

It also is likely that when considering the steam-water cycle, expenditure on water conditioning equipment at the front end will result in savings in chemical treatments, fuel, and loss of good-quality hot water at the back end ... 

The term cold end commonly refers to the back-end convection area of the boiler system where an economizer, air-heater, or ID fan may be found, together with the stack and (with high dust-burden flue gases) possibly a cyclone scrubber or electrostatic precipitator. Any fuel treatment applied in this area may be considered to act as a postcombustion additive. 

A typical semiconductor device (found in the back-end of the line or the interconnects) consists of a layer of glass followed by a sputtered layer of titanium, which is thermally treated to form a titanium silicide. Next, a layer of titanium nitride is deposited on top of the silicide and on the sidewall of the contacts by sputtering or by MOCVD (see Fig. 13.3 in Ch. 13).P ]P ] This layer of TiN acts as a diffusion barrier and an adhesion promoter. It is followed by the main interconnect, which is an aluminum-copper alloy, in turn followed by another layer of TiN, which acts as adhesion and antireflecting layer. 

Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.12). The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride enrichment of the uranium-235 isotope conversion of the enriched product to uranium oxides and fabrication into reactor fuel elements. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 

This initial medicinal chemistry ELN was followed into the market by products such as the Arthur suite from Synthematix and the iELN system from Intellichem (both since acquired by Symyx), which were more oriented at reaction planning and process chemistry. The heavyweight of the traditional cheminformatics companies, Elsevier MDL, also released a system, called Elan, which combined a Word-based front end with their well-known ISIS chemical technology on the back end (Fig. 9.3). 

Separation processes, as could be seen from Figure 2.1, position themselves at the back end of the sequence in operations in the mineral processing flowsheet. The front-end operations has been found virtually to terminate with the liberation or the size-reduction processes involving crushing and grinding. It is important to limit the amount of size reduction to that at which adequate liberation is accomplished. The term adequacy is related to the cost involved in comminution and to performance of the concentration methods that follows. The concentration is obtained by separation processes which rely on differences in the properties of the particles, the physical and physico-chemical characteristics of minerals. In this context, it will only be relevant to refer to Table 2.5 which presents a summary of the processes along with the properties of the minerals that are exploited. 

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