Computer programming

On the other hand, the use of special subroutines for doing specific calculations is highly recommended. The book by Franks (Motkling and Simulation in Chemical Engineering, John Wiley and Son, Inc., 1972) contains a number of useful subroutines. And of course there are usually extensive libraries of subroutines available at most locations such as the IMSL subroutines. These can be called very conveniently from a user s program. 

A comprehensive discussion of computer programming is beyond the scope of this book. I assume that you know some computer programming language. All the examples will use FORTRAN since it is the most widely used by practicing 

Many people get all excited about including extensive comment statements in their code. Some go so far as to say that you should have two lines of comments for every one line of code. In my view this is ridiculous If you use symbols in your program that are the same as you use in the equations describing the system, the code should be easy to follow. Some comment statements to point out the various sections of the program are fine. 

For example, in distillation simulations the distillate and bottoms composition should be called XD(J) and XB(J) in the program. The tray compositions should be called X(NJ), where IV is the tray number starting from the bottom and J is the component number. Many computer scientists put all the compositions into one variable X(NJ) and index it so that the distillate is X(1J), the top tray is X(2J), etc. This gives a more compact program but makes it much more difficult to understand the code. 

Subroutine arguments and COMMON statements can also be troublesome. Always make sure that the calls to a subroutine use the correct sequence of arguments and that all COMMON statements in all subroutines are exactly the same as that in the main program. 

An essential request for any scientific product is reproducibility. In this perspective, access to the source code should be as wide as possible, and its analysis, evaluation, and modification should be as easy as possible. The most widely used symbolic language in the scientific milieu is called Fortran, short for Formula Translator. Its syntax and the corresponding compiler were originally developed in the 1950s by a team of programmers at IBM [8], and, despite sometimes harsh criticism, it is still the language of choice for dealing with mathematical expressions. In Fortran, the instmction for an 

On the hardware side, other speedup strategies are essentially based on the simple idea that since hardware is becoming less and less expensive, one may use many processing units instead of only one and split up the job. Parallelization and clustering strategies belong to this family either the central processors carry out many calculations at the same time, as in parallel machines, or physically separate machines actually load different parts of the total job, as happens in clusters, which may nowadays contain up to 50-100 computers. In both cases the big problem is the synchronization 

The absolutely safest strategy is to load the same software on to separate machines and to split manually the job from the very beginning. This can be done in discontinuous jobs, such as when one has to calculate the lattice energy of 100 crystals and, with two machines, put 50 calculations on each. It cannot work in molecular dynamics simulations, where the trajectory must be continuous. 

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The calculation of vapor and liquid fugacities in multi-component systems has been implemented by a set of computer programs in the form of FORTRAN IV subroutines. These are applicable to systems of up to twenty components, and operate on a thermodynamic data base including parameters for 92 compounds. The set includes subroutines for evaluation of vapor-phase fugacity... 

Discusses the thermodynamic basis for computer calculations for vapor-liquid equilibria computer programs are given. Now out of date. 

VPLQFT is a computer program for correlating binary vapor-liquid equilibrium (VLE) data at low to moderate pressures. For such binary mixtures, the truncated virial equation of state is used to correct for vapor-phase nonidealities, except for mixtures containing organic acids where the "chemical" theory is used. The Hayden-0 Connell (1975) correlation gives either the second virial coefficients or the dimerization equilibrium constants, as required. 

On the other hand, this approach has a number of advantages. Many different design options can be considered at the same time. Also, the entire design procedure can be accommodated in a computer program capable of producing designs quickly and efficiently. 

Given the estimate of the reactor effluent in Example 4.2 for fraction of methane in the purge of 0.4, calculate the.actual separation in the phase split assuming a temperature in the phase separator of 40°C. Phase equilibrium for this mixture can be represented by the Soave-Redlich-Kwong equation of state. Many computer programs are available commercially to carry out such calculations. 

Using computer programs compiicates the problem because the calculation accuracy is never given for commercial reasons. Furthermore, the ways in which the methods are executed are not explicit and the data banks are often considered secret and inaccessible. 

Bostrom, A., UTDefect - a computer program modelling ultrasonic NDT of cracks and other defects, SKI report 95 53, Stockholm, 1995. 

A computer program was written to perform all the calculation. It is found that the three-element viscoelastic model provides reasonable estimation of the behavior of the polyvinyl chloride material during the impact... 

In addition, the mirrors are adjustable, so that unimportant areas can be ignored. Light re-emmited from the surfaee is detected, and the detector signal is transmitted to a computer programmed with acceptable deviation levels for comparison with a reference component. Tolerance levels can vary for different areas of the same test piece they may, for example, be higher on a ground section than on adjacent unmachined areas. 

Unfortunately, the supennolecule approach [81, 82] is full of teclmical diflSculties, which stem chiefly from the very small magnitude of the interaction energy relative to the energy of the supennolecule. Even today, a novice would be ill-advised to attempt such a computation using one of the black-box computer programs available for perfonning ab initio calculations. 

In the experimental and theoretical study of energy transfer processes which involve some of the above mechanisms, one should distingiush processes in atoms and small molecules and in large polyatomic molecules. For small molecules a frill theoretical quantum treatment is possible and even computer program packages are available [, and ], with full state to state characterization. A good example are rotational energy transfer theory and experiments on Fie + CO [M] ... 

Enonnous numbers of chemical shifts have been recorded, particularly for FI and Many algoritlnns for the prediction of shifts have been extracted from these, so that the spectra of most organic componnds can be predicted at a useful level of accuracy, usmg data tables available in several convenient texts [12, F3,14 and 15]. Alternatively, computer programs are available that store data from 10 -10 spectra and then use direct... 

F) EFFICIENT AND WIDELY DISTRIBUTED COMPUTER PROGRAMS EXIST FOR CARRYING OUT ELECTRONIC STRUCTURE CALCULATIONS... 

Nesbet R K 1963 Computer programs for eleetronie wave-funetlon ealeulatlons Rev. Mod. Rhys. 35 552-7... 

Chemists have been used to drawing chemical structures for more than a hundred years. Nowadays, structures are not only drawn on papei but they are also available in electronic form on a computer for publications, for presentations, or for the input and outptit with computer programs. For these applications, well-known software such as ISIS/Draw (MDL [31] or ChemWindow (Bio-Rad Sadtier [32]) arc used (see Section 2,12), The structures generated with these programs arc... 

Any one of these additivity schemes can be used for the estimation of a variety of thermochemical molecular data, most prominently for heats of formation, with high accuracy [13]. A variety of compilations of thermochemical data are available [14-16]. A computer program based on Allen s scheme has been developed [17, 18] and is included in the PETRA package of programs [19]. 

The Rekker approach is still used with revised Z/ systems, e.g., in the software program Z/SYBYL [8]. Over recent decades various other substructure-based approaches have been developed that are mostly implemented and available as computer programs. 

Molecular modelling used to be restricted to a small number of scientists who had access to the necessary computer hardware and software. Its practitioners wrote their own programs, managed their own computer systems and mended them when they broke down. Today s computer workstations are much more powerful than the mainframe computers of even a few years ago and can be purchased relatively cheaply. It is no longer necessary for the modeller to write computer programs as software can be obtained from commercial software companies and academic laboratories. Molecular modelling can now be performed in any laboratory or classroom. 

Dalby A, J G Nourse, W D Hounshell, A K I Gushurst, D L Grier, B A Leland and J Laufer 1991 Description of Several Chemical Structure File Formats Used by Computer Programs Developei at Molecular Design Limited, journal of Chemical Information and Computer Science 32 244-255. 

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