CONTENTS Semiempirical Methods

HKL [1] make the point that calculations are not just alternatives to experiment, as Dewar thinks, but can also illuminate experiment. In effect, they say that calculations are not only another way to get numbers, but can provide insight into physical processes. Their contention that such insight comes from ab initio, not from semiempirical, methods (which obscure the physical bases of their success and failure) seems to be justified, because in SE methods the fundamental physical entities have been deliberately subsumed into parameters designed to give the right, or rather the best, answers. 

Molecular orbital computational analysis by PM3 CI UHF semiempirical methods have been used to support the contention that preferable HSOMO-LUMO interactions produce a favored biradical and explain the site selectivity in the sensitized photochemical [2 + 2] cycloadduct formation of 2-pyrones with electron-deficient ethylenes <92BCJ354>. The lowest ionization energies, dipole moment, and dominant electronic configurations of a 5-methylidenated version of 7-nitroso-oxazolo[4,5- ]cyclopenta[e]pyrimidine of unknown origin were calculated by the ADC(3) ab initio method <92CPHii>. An extensive semiempirical and ab initio investigation into the mechanism of oxidation of methanol by PQQ is cited in Section 7.22.12.4. 

A search of Current Contents showed that in 1999, AMI was the most widely used semiempirical method and PM3 was second. 

Many other semiempirical methods can be reported, like the procedure suggested by Koros and Novak [42] to test the importance of intrapellet or extrapellet mass transfer resistance by varying the active metal content in the catalyst using the same hydrodynamic conditions. 

In our group, Chen Hairong did a lot of works in the study of dynamic characteristics of PEMs. She established the relevant dynamic characteristics of proton exchange membrane by semi-semiempirical method mechanism. In the past, the proton exchange membrane water content is usually calculated by the empirical... 

Energy calculation and minimization One of the fundamental properties of molecules is their energy content and energy level. Three major theoretical computational methods of their calculation include empirical (molecular mechanics), semiempirical, and ab initio (quantum mechanics) approaches. Energy minimization results in geometry optimization of the molecular structure. 

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