Semiempirical calculations, advantages

Solids and surfaces can be modeled as large clusters, as embedded clusters, and as infinite periodic systems. Each of these strategies has been used in conjunction with semiempirical MO methods. The most straightforward semiempirical approach employs clusters of increasing size, taking advantage of the fact that semiempirical calculations can be extended easily to systems with more than 1000 atoms which facilitates convergence studies to the bulk limit. Semiempirical work in this field has been reviewed recently [45]. 

The two approximate SCF methods just mentioned are advantageous because of their simphcity and rapid convergence. They have proved useful with both semiempirical and ab initio treatments (vide infra). Their approximate nature is not associated, at least in semiempirical calculations, with any drawback, inasmuch as we found in PPP-like and CNDO treatments 13.21,22) that the methods of Roothaan and of Lon-... 

Semiempirical methods, for example, MNDO. AMI, and PM3. are simplifications of ab initio molecular orbital theory and employ empirically determined parameters in essence, they only differ in the approximations being made. These methods involve adjustable parameters associated with molecular properties that are calibrated against experimental data. The chief advantage of semiempirical calculations over ab initio calculations is that they are several orders of magnitude faster. Thus, calculations for systems of up to c. 200 atoms are currently possible, whereby with ab initio methods, the limit is a moderately sized molecule (about 50 atoms at the time of writing), if rational results are to be obtained. Frequently, semieinpirical methods have proved to be the computational procedures of choice for studying relatively large molecules. 

The level of accuracy that can be achieved by these different methods may be viewed as somewhat remarkable, given the approximations that are involved. For relatively small organic molecules, for instance, the calculated AGsoivation is now usually within less than 1 kcal/mole of the experimental value, often considerably less. Appropriate parametrization is of key importance. Applications to biological systems pose greater problems, due to the size and complexity of the molecules,66 156 159 161 and require the use of semiempirical rather than ab initio quantum-mechanical methods. In terms of computational expense, continuum models have the advantage over discrete molecular ones, but the latter are better able to describe solvent structure and handle first-solvation-shell effects. 

If it could be shown that ab initio SCF calculations were effective in at least certain connections, they would of course present obvious advantages in that they are based on a rigorous solution of a specific mathematical problem and so involve no parameters. Consequently they can be applied equally well to systems of all kinds, containing any elements. Semiempirical treatments are limited to systems for which parameters have been determined. Even if computation time presents an inseparable barrier to ab initio treatments of systems large enough to be of chemical interest, such calculations for simpler systems might prove useful as an aid in developing semiempirical treatments. 

In summary, semiempirical methods such as AMI or PM3 have the advantage of being computationally very fast and allowing large molecules to be computed with minimal computer resources. The semiempirical methods are not nearly as accurate as the ab initio methods or even density functional methods. Some disadvantages of these methods include the following (1) they can be applied only to molecules containing elements for which they have been parameterized, (2) the errors are less systematic than at an ab initio level of calculation, and (3) semiempirical methods depend on the availability of accurate experimental data (or reliable ab initio data). 

What do you think are the advantages and disadvantages of parameterizing semiempirical methods with data from ab initio calculations rather than from experiment Could a SE method parameterized using ab initio calculations logically be called semi empirical ... 

An alternative to the molecular mechanical approach is the quantum mechanics (QM) framework. Much of work has been devoted in the past decades to establish and improve QM methodologies ranging from simple semiempirical methods to high-level correlated ab initio methods [20,21,29], The substantial advantage of quantum mechanical calculations is the inclusion of all n-body effects, including charge transfer and polarisation. Furthermore, bonds are automatically formed and broken as necessary along the simulation. 

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