Semiempirical MO theory

Thiel, W., 1998. Thermochemistry from Semiempirical MO Theory, In Irikura, K. K. Frurip, D. J. Eds., 1998. Computational Thermochemistry. American Chemical Society, Washington, DC. 

Historically, two philosophies began to emerge at this stage with respect to how best to make further progress. The first philosophy might be summed up as follows The HF equations are very powerful but still, after all, chemically flawed. Thus, other approximations that may be introduced to simplify their solution, and possibly at the same time improve their accuracy (by some sort of parameterization to reproduce key experimental quantities), are well justified. Many computational chemists continue to be guided by this philosophy today, and it underlies the motivation for so-called semiempirical MO theories, which are discussed in detail in the next chapter. 

It was Hellmann (1935) who first proposed a rather radical solution to this problem -replace the electrons widi analytical functions that would reasonably accurately, and much more efficiently, represent the combined nuclear-electronic core to the remaining electrons. Such functions are referred to as effective core potentials (ECPs). In a sense, we have already seen ECPs in a very crude form in semiempirical MO theory, where, since only valence elections are tieated, the ECP is a nuclear point charge reduced in magnitude by the number of core elections. 

To speed this process up further for very large systems, one can make some further assumptions entirely analogous to those found in semiempirical MO theory. In particular, one may assume... 

L. L. Lohr and W. N. Lipscomb, Inorg. Chem., 2, 911 (1963). (An approach to Jahn-Teller effects via semiempirical MO theory.)... 

Semiempirical MO Theory Caveats and Performance. The same basic theoretical assumptions are made as in ab initio theory. 

This qualitative model, based on semiempirical MO theory, focuses entirely on the so-called electronic effects, as the A—H bonding orbital interactions are often called. However, steric repulsion (i.e., the destabilizing orbital interactions) between the hydrogen substituents in AH3 is just as important in the interplay of mechanisms that determine whether the molecule adopts a planar or a pyramidal shape. In fact, as will become clear from the following discussion, which is based on a Kohn-Sham DFT study at the BP86/TZ2P level,107 108 steric repulsion turns out to be the decisive factor in determining the pucker of our example.133... 

In the years since 1990, there have been some major new developments in semiempirical methodology which will be summarized in Section 21.4. To put these developments into perspective, the basic features of the established semiempirical methods (see above) will be outhned briefly in the following Section 21.3 for a more detailed account, the reader is referred to the available books [30-35] and review articles [36-45] on semiempirical MO theory. 

In this section, we summarize advances in semiempirical MO theory that have been made in the years since 1990 in seven areas. In most of these cases, there are current activities aiming at further methodological improvements, and the discussion in this section should thus give an indication on where the field is moving. It should be stressed, however, that the selection of topics is subjective and not intended to be comprehensive. 

The application of molecular orbital theory to predict structures and properties of C species started with the pioneering work of Pitzer and dementi in 1959 [225]. By using semiempirical MO theory, these authors found the cumulenic linear conformation to be the most stable geometry for except possibly for very large values of n. They predicted that the n-odd clusters would have closed-shell structures and lower energies than the singlet n-even species. Cyclic molecules were found to be unimportant under most conditions. 

Semiempirical MO theories fall into two categories those using a Hamiltonian that is the sum of one-electron terms, and those using a Hamiltonian that includes two-electron repulsion terms, as well as one-electron terms. The Hiickel method is a one-electron theory, whereas the Pariser-Parr-Pople method is a two-electron theory. 

It is interesting that earlier calculations based on the geometry-optimized LHS semiempirical MO theory on the series shown in Table 7.6 provided a decreasing trend for the gap across the series, because all three appear as aromatic at that level of theory (Ktirti et al 1993). The level crossing does not emerge yet in the first three members of the series. This indicates two general points worth considering (1) the... 

Beck B, Clark T (1998) Some biological applications of semiempirical MO theory. Perspect Drug Discov 9-11 131-159... 

Green s Function Ionization Potentials in Semiempirical MO Theory... 

GREEN S FUNCTION IONIZATION POTENTIALS IN SEMIEMPIRICAL MO THEORY 1191... 

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