Semiempirical codes

Three popular semiempirical programs, AMP AC, AMSOL, and MOPAC, are actually derivations of the same original code. AMP AC 1.0 and MOPAC 3.0 were both created from Version 2.0 of MOPAC. AMSOL was derived from... 

Generally, the behavior of a gas (or vapor) cloud during dispersion can be either buoyant or gravity driven. The former is regarded with heavier-than-air gases and the later with lighter-than-air gases. Currently, dispersion estimations can be performed via the use of semiempirical one-dimensional models (the so-called box models) and the CFD codes. 

Nowadays, more than 4000 types of descriptors are known.17 There exist different ways to classify them. With respect to the type of molecular representation used for their calculations—chemical formula, molecular graph, or spatial positions of atoms—one speaks about ID, 2D, and 3D descriptors, respectively. Descriptors can be global (describing the molecule as a whole) and local (only selected parts are considered). One could distinguish information-based descriptors, which tend to code the information stored in molecular structures, and knowledge-based (or semiempir-ical) descriptors issued from the consideration of the mechanism of action. Most of those descriptors can be obtained with the DRAGON, CODESSA PRO, and ISIDA programs. 

The reliability of the newly-developed rigid-rotor potential was tested by means of close-coupling calculations of rotational state-to-state integral cross sections. The MOLSCAT code was used [66]. The results were compared with those obtained using the semiempirical potential of Buck, and with the available experimental data. 

Generalized Reaction Fields from Surface Charge Densities Ab initio formulations of the PCM model discussed earlier, undertaken primarily by Tomasi and co-workers (see, e.g.. Refs. 72, 73, 266, 267), have very recently been implemented into four different semiempirical packagcs.- - Available codes include MOPAC,30o,325 a locally modified s version of MOPAC, oo and VAMP.302 While the model used by Negre et al. o NDDO Hamiltonians follows exactly the derivation of Equations [23] and [27], those of Wang and... 

In addition to ab initio methods, developments for semiempirical methods are being investigated on parallel machines. Notable in this regard are MOPAC > 20 (at the San Diego Supercomputer Center i EPSRC Dares-bury Laboratory ) and AMSOL.122 expected, many of the problems encountered with ab initio codes are also found with these programs. 

The original procedure (Miertus et al., 1981) was expressed in the restricted Hartree-Fock (HF) formalism. Extensions to other levels of the quantum theory are easy, and there are versions of the PCM program accepting UHF, ROHF, MPn, CASSCF, SDCI, MRCI, and CASSCF-CI levels of quantum molecular theory. The extension to semiempirical quantum methods has been elaborated by several groups. We quote here Miertus et al. (1988) for CNDO methods, and Luque and Orozco (Luque et al., 1993 1995 Negre et al., 1992) for PM3 and AMI version. Both of them follow this approach. The computational codes are of public domain. The PM3 version has been included in MOPAC package (Stewart, 1990). 

The recommended approach to modeling is to create models based on fundamental balances (of mass, species, energy, population) and basic kinetics and use them to build a complete model of the precipitator, as shown in earlier sections. Such a set of equations is known as a physical or a mechanistic model. Complete physical models are difficult to create and solve because they require identification in advance of all physical and chemical subprocesses, properties, and parameters. That is why the semiempirical models of a form similar to the complete physical models (but usually simpler) and with fewer equations are often used for scaling up. Parameters of such models are often given in lumped form, some of them fitted to available experimental data obtained from the small-scale system. Such a model can be useful for scaling up, but one cannot be sure that the scale-up will be completely correct because there is no guarantee that the model contains the complete mechanism (88). However, scale-up errors should be smaller than in the case of purely empirical models. CFD codes that are based on reasonable simplifications (closures) regarding their accuracy can be placed between the physical and semiempirical models their application was demonstrated earlier. 

In addition, we will concentrate on results from programs that are generally available and therefore have reproducible results. There does, however, exist a potential problem with the reproducibility of results quoted here. With the passage of time, new parameters do arise in semiempirical methods updates. These updates are not always documented, and modified versions of computer code do circulate containing parameters that have not been carefully examined. Sometimes these new parameters repre.sent improvements, and sometimes they do not. 

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