Semiempirical methods perspectives

A common application of the direct calculation of molecular energy is the study of organic reaction mechanisms. You can investigate the energies of different potential intermediates, species not easily studied by experiment. A review by Thiel lists many such 39. Thiel, W. Semiempirical Methods Current Status and Perspectives Tetrahedron, 44 7393, 1988. 

Thiel, W., Semiempirical methods current status and perspectives, Tetrahedron, 44, 7393-7408, 1988. 

W. Thiel, Tetrahedron, 44, 7393 (1988). Semiempirical Methods Current Status and Perspectives. 

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. 

Seabra GD, Walker RC, Roitberg AE (2009) Are current semiempirical methods better than force fields A study from the thermodynamics perspective. J Phys Chem A 113 11938-11948... 

Thiel W 1996 Perspectives on semiempirical molecular orbital theory New Methods in Computationai Quantum Meohanios (Adv. Chem. Phys. XCiti) ed I Prigogine I and S A Rice (New York Wiley) pp 703-57 Earlier texts dealing with semi-empirical methods include ... 

At present the chemist has a wide variety of quantum chemical methods available, and it is difficult to keep the numerous modifications of only a few basic statements in perspective, even for the insiders. All methods can be divided into two basic categories the ab initio calculations and the semiempirical calculations. The ab initio as well as the semiempirical treatments of chemical problems were developed proportional to the computers because... 

At the time of this writing, it must be conceded that there have been no fundamental principles-based mathematical model for Nafion that has predicted significantly new phenomena or caused property improvements in a significant way. Models that capture the essence of percolation behavior ignore chemical identity. The more ab initio methods that do embrace chemical structure are limited by the number of molecular fragments that the computer can accommodate. Other models are semiempirical in nature, which limits their predictive flexibility. Nonetheless, the diversity of these interesting approaches offers structural perspectives that can serve as guides toward further experimental inquiry. 

The simulations in this study were done using the semiempirical MO method PM3(Stewart, 1989 Stewart, 1989 Stewart, 1991) within the context of the QM/MM coupled potential recently implemented within our lab.(Hartsough and Merz, 1995) A modified version of MOP AC 5.0 (Besler, Merz et al., 1990 Merz and Besler, 1990), was used for the QM calculations and the driver MD program was a customized version of AMBER (Pearlman, Case etal., 1991). The PM3 Hamiltonian was selected because of its superior correlation with experimental heats of formation for the compounds studied herein (Stewart, 1989). From a methodological perspective, AMI (Dewar, Zoebisch et al., 1985), or MNDO (Dewar and Thiel, 1977), or any other LCAO based method could have been used. 

In the second-order methods we have described, the choice of coordinate system was not made explicit. Prom a quantum-chemical perspective, analytical derivatives are most conveniently computed in Cartesian (or symmetry-adapted Cartesian) coordinates. Indeed, second-order methods are not particularly sensitive to the choice of coordinate system and second-order implementations based on Cartesian coordinates usually perform quite well. As we discussed above, however, if the Hessian is to be estimated empirically, a representation in which the Hessian is diagonal, or close to diagonal, is highly desirable. This is certainly not true for Cartesian coordinates some set of internal coordinates that better resemble normal coordinates would be required. Two related choices are popular. The first choice is the internal coordinates suggested by Wilson, Decius and Cross [25], which comprise bond stretches, bond angle bends, motion of a bond relative to a plane defined by several atoms, and torsional (dihedral) motion of two planes, each defined by a triplet of atoms. Commonly, the molecular geometry is specified in Cartesian coordinates, and a linear transformation between Cartesian displacement coordinates and internal displacement coordinates is either supplied by the user or generated automatically. Less often, the (curvilinear) transformation from Cartesian coordinates to internals may be computed. The second choice is Z-matrix coordinates, popularized by a number of semiempirical... 

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