MNDO model strengths

There are known strengths ind weaknesses of each semiempirieal method. The AMI and PM3 models only include s- and p-funetions, which limits their u.sefulncss for most elements of the periodic table that require d-orbitals. Many of the.se elements, however, arc not typically found in most drug-like molecules. More recent advances with MNDO/d include the incorporation of d-functions in the NDDO (neglect of diatomic differential overlap) model."" ""... 

As the name implies, semi-empirical calculations simplify the computational problem presented by the Schrodinger equation. Semi-empirical approaches eliminate some of the direct number crunching involved in ab initio calculations and instead replace portions of the calculation with values that may be taken from experimental data or other calculations that are parameterized to agree with empirical data. There is a variety of semi-empirical schemes that differ in the types of parameterizations that are made. Three common semi-empirical methods that are included in Spartan02 are called AMI, PM3, and MNDO. Each has strengths and weaknesses depending on the specific molecular environment one wishes to model. 

There are, however, two major disadvantages to the finite field procedure. The first is that it is limited to static fields, and hence the method does not give values that can be directly related to most experiments. This is not necessarily a serious problem because as a rule only an estimate or information on relative properties is needed. The second disadvantage is that, like all numerical derivative schemes, the finite field procedure may exhibit severe numerical problems. In the example of this shown in Figure 4, the calculated y value for a biphenyl molecule obtained by a semiempirical method (MNDO) is plotted as a function of the arbitrary choice of the base field strength F used in Eqs. [28] and [29]. The correct value within the model is the limit as F - 0. Note the large dependence of the choice of F in the calculation. As the base field increases, the calculated properties based on E(F) and p(f) deviate more and more from the... 

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