Semiempirical Calculations on Larger Molecules

Semiempirical molecular orbital calculations have gone through many stages of refinement and elaboration since Pople s 1965 papers on CNDO. Programs like PM3, which is widely used in contemporary research, are the cumulative achievement of numerous authors including Michael Dewar (1977), Walter Thiel (1998), James Stewart (1990), and their coworkers. 

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SEMIEMPIRICAL CALCULATIONS ON LARGER MOLECULES The spin eigenfunctions are orthogonal... 

The extent of approximation and parameterization varies with the different MO methods. As computer power has expanded, it has become possible to do MO calculations on larger molecules and with larger basis sets and fewer approximations and parameters. The accuracy with which calculations can predict structure, energy, and electron density has improved as better means of dealing with the various approximations have been developed. In the succeeding sections, we discuss three kinds of MO calculations (1) the Htickel MO method, (2) semiempirical methods, and (3) ab initio methods, and give examples of the application of each of these approaches. 

This restriction rules out all discrete models exclusively based on semiempirical force fields, leaving among the discrete models the MC/QM and the MD/QM procedures, in which the second part of the acronyms indicates that the solute is described at the quantum mechanical (QM) level, as well as the full ab initio MD description, and some mixed procedures that derive the position of some solvent molecules from semiclassical simulations, replace the semiclassical description with the QM one, and repeat the calculation on these small supermolecular clusters. The final stage is to perform an average on the results obtained with these clusters. These methods can be used also to describe electronic excitation processes, but at present, their use is limited to simple cases, such as vertical excitations of organic molecules of small or moderate size. This limitation is due to the cost of computations, and there is a progressive trend toward calculations for larger systems. 

For calculations on oligonucleotides and small polypeptides that form an active site, as well as any small molecules that will interface with the site, ab initio or DFT methods are good choices. For larger chains, semiempirical methods are preferred. 

As a result, informations of comparable quality may be obtained from both types of calculations, regardless of the different definitions adopted for the charge densities. For these reasons in the present chapter electron density distributions in cumulenes are discussed on the basis of semiempirical CNDO/S data, which allow a consistent and facile treatment also for larger molecules. Notably, CNDO/S electron densities do not correlate with CNDO/2 or INDO electron densities (Section I1.G.2). 

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