Quantum semiempirical calculations

Lee T, York D, Yang W (1996) Linear-scaling semiempirical quantum calculations for macromolecules. J Chem Phys 105(7) 2744-2750... 

At the present time, semiempirical quantum calculations applied to a restricted range of fluorine compounds cannot be said to be more successful than other empirical methods, such as Sanderson s, although they yield other molecular properties in addition to enthalpies. 

A complete treatment must include all the geometrical modifications occurring during the reaction that make up the total energy of the TS. This can be approximated by semiempirical quantum calculations or by molecular mechanics. 

For this particular example, the bisection by plane (Figure 4) is better than the oct-tree (Figure 3), because in the former case, the > and sets are well separated, while in the latter there are no well-separated sets. For some problems, such as the inclusion of solvation effects in linear-scaling semiempirical quantum calculations, the highly irregular surfaces of biological molecules make the recursive bisection by planes ideal. 

Conformations of 4-oxo-l,6,7,8,9,9n-hexahydro-4//-pyrido[l,2-n]pyrimi-dine-3-carboxylates and -3-carboxamides were studied by semiempirical quantum chemical calculations at the AMI level (97H(45)2175). While 1-methyl-9n-unsubstituted derivatives adopt a cA-fused conformation, 9a-ethoxy-1-methyl derivatives adopt a trans-fu ed one to avoid a serious non-bonding interaction between 9n-ethoxy and 1-methyl groups, which would be present in an alternative cA-fused conformation. 

M.S. Daw and M.I. Baskes, Semiempirical, quantum mechanical calculation of hydrogen... 

The last point has been studied more quantitatively for the electrolyte LiOjCCH Fy (x+y=3) / DMSO [97,105], Semiempirical quantum-mechanical calculations with the help of MOP AC [143] show that the mean electron density at the oxygen atoms q(0) decreases for these acetates by about 0.1 unit with increasing fluorine content of the anion [97]. As a consequence ... 

When quantum calculations, at the ab initio and at the semiempirical level, gained foot in the realm of chemistry, a steadily increasing number of experimentalists began to use quantum calculations as a supplement in the exposition of their findings. In many case this was - and still is - nothing more that an ornament, like decorations on a cake. This use of quantum chemistry has been, in general, harmless, because results in contrast with experimental evidence have been rarely published, and this production may be considered now as a sort of advertising for the new-born computational chemistry. A more serious use of the facilities offered by the computational techniques is done by scientists provided of... 

Section 2 mainly focuses on the current efforts to improve the accuracy of quantum calculations using simplified empirical model forms. McNamara and Hillier, in Chapter 5, summary their work on improving the description of the interactions in biological systems via their optimized semiempirical molecular models. Piquemal and co-workers present recent advances in the classical molecular methods, aiming at better reproduction of high-level quantum descriptions of the electtostatic interactions in Chapter 6. In Chatper 7, Cui and Elstner describe a different semiempir-... 

The heteropolar transition state was also predicted by semiempirical quantum-chemical calculations (78T2315). The ground-state properties of... 

In Section II,B,8 we discussed the question of determining site densities using high-conversion data. We developed a method applicable in the inter-conversion of three isomers when there is a common surface complex for the three possible reactions. We have tested this method using the conversion of 1-butene to cis- and rrans-2-butene over silica-alumina, a system that, according to Hightower and Hall, proceeds through a common surface complex (111). Their conclusion has been confirmed experimentally (112) and by semiempirical quantum-chemical calculations (113). 

SemiChem products are available at http //www.semichem.com. AMPAC , available as a stand-alone product with Windows-based and workstation level interfaces, is a semiempirical quantum mechanical program featuring SAMI, AMI, MNDO, MNDO/d, PM3, MNDO/C, MINDO/3 semiempirical methods. AMPAC also includes a graphical user interface (GUI) that builds molecules and offers full visualization of results. The SYBYL/Base program offered by Tripos, Inc. (products described below) provides an interface with interactive graphing and structural display tools that can be used to access AMPAC s calculation tools. 

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