Rule-based scheme

The two most important representation schemes are the rule-based scheme and the frame-based or object-oriented scheme. 

The most popular representation scheme in expert systems is the mle-based scheme. In a rule-based system the knowledge consists of a number of variables, also called attributes, to which a number of possible values are assigned. The rules are the functions that relate the different attributes with each other. A mle base consists of a number of If... Then... mles. The IF part contains the conditions that must be satisfied for the actions or conclusions in the THEN part to be valid. As an example, suppose we want to express in a mle that a compound is unstable in an alkaline solution if it contains an ester function. In a semi-formal way the mle can be written ... 

Second-order and third-order results often bracket the true correction to pF - Three schemes that scale the third-order terms in various ways are known as the Outer Valence Green s Function (OVGF) [8], In OVGF calculations, one of these three recipes is chosen as the recommended one according to rules based on numerical criteria. These criteria involve quantities that are derived from ratios of various constituent terms of the self-energy matrix elements. Average absolute errors for closed-shell molecules are somewhat larger than for P3 [31]. 

Although the data shown in Fig. 3 clearly indicate that this reaction proceeds via a chain mechanism, it is not clear how this reaction is initiated. Possible mechanisms that must be considered include exciton-based schemes involving surface localized holes that facilitate the attack of the alkene nucleophile (such as proposed for the white light alkylation of porous silicon [23]). Flowever, given the low efficiency of the initiation process it is difficult to completely rule out the role of photogenerated radicals from impurities in solution, even when high purity reactants are used. 

In addition, there have also been a number of efforts to find more efficient ways to categorise and enumerate compounds. One approach to defining the relationship of compounds to each other uses a chemical space defined by a set of 500 compounds described with 60 molecular descriptors onto which novel compounds can then be mapped.116 Another approach that has been described uses a rule-based classification scheme to define the common scaffold or core of compounds.117 This approach has been taken further to develop a scoring function to allow the prioritisation of compound libraries for testing.118... 

Most of the applications discussed here were based on algorithmic systematic generation approaches to process synthesis. There also exist a number of heuristic rule-based or expert-systems-based approaches including PIP for total flowsheet synthesis, SPLIT for separation schemes (Wahnschafft et ai, 1991),... 

Knowledge Representation for IQAP. Three knowledge representation schemes, i.e., relational, rule-based and frame-based, and constraint-based, were examined for developing IQAP s knowledge base. For a general description of some knowledge representation schemes (4). 

To summarize, a rule-based or frame-based system could provide a reasonably flexible tool to the Superfund Program, which may at times need to specify and utilize small modifications of a standard, such as the CLP QC criteria. Such a system could select a CLP method and its associated QC criteria based on the inputs (Section 3.1), modify the QC limits for certain analytes and/or samples, and specify modifications in the rules of thumb used for validation. However, the potential for an inordinately large number of rules, this apparently simple solution has the practical drawback of embedding conflicts among the rules which will be difficult to debug and modify. This is contrary to the way in which experts think and consequently, this scheme is deemed to be unsuitable. 

The UFF force field represents still another type of force field in that it is neither parameterized in the classical way nor based on ab initio quantum chemical calculations. Instead, the parameters in UFF are evaluated from general rules based on the elements, their hybridization and connectivity. The reason for this choice of parameterization scheme is that the goal of the UFF force field is to extend the calculations to the entire periodic table, making the large number of parameters required for such a task untenable by traditional prescriptions. 

The retrosynthetic plan (Scheme 10) indicates that the dienophile would be a carboxyketene equivalent, CII, which implies the diastereofadal selectivity would be controlled entirely by the diene component Cl. At the outset of this work, there were few examples where chiral dienes had been employed in intermolecular Diels-Alder reactions upon which the investigators could rely for precedents [28, 29]. However, seminal studies had been undertaken by Franck and co-workers who reported that the sorbaldehyde diene CHI reacted with N-phenylmaleimide to give a 7.3 1 mixture of cycloadducts CIV and CV (Scheme 10b) [29]. Thus, the major product resulted from a like process (i.e., preferential attack from the r -face) and these workers developed a set of rules based on their experimental observations. 

M. Anastasio, M. Kupinski, and R. Nishikawa . Optimization and FROC analysis of rule-based detection schemes using a multiobjective approach. IEEE Tmnsactions on Medical Imaging, 17 1089-1093,1998. 

DAA [Kowalski84] is a rule-based expert system approach to synthesis. The choice of functional units, however, is determined algorithmically by BUD [McFarland86a], which estimates area and performance for several design options, and chooses the best. The DAA program chooses registers and an appropriate interconnection scheme for the design. 

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