The semiempirical approach to chemistry

Abstract

Two kinds of quantum mechanical treatment are needed in chemistry: First, a treatment accurate enough to provide definitive solutions of chemical problems, in particular, the prediction of reactions and reaction mechanisms, without reference to experiment. As yet, treatments of this degree of accuracy are restricted to a few small atoms and molecules. Second, a treatment that chemists can use as a practical aid to their own research, on the same basis as mass spectrometry or NMR. Here, the need has been largely met by the series of progressively better semiempirical methods, in particular, those developed by our group. Since only the best and most expensive ab initio procedures are superior to ours and since the computing time they need is far greater, their use as a practical aid in chemistry is restricted. Our procedures, therefore, serve as a useful supplement to ab initio ones in areas where the latter cannot be applied effectively, as the leading ab initioists now fully recognize. Since the semiempirical approach is capable of almost unlimited further development, this situation is likely to continue in the foreseeable future. 0 1992 John Wiley & Sons, Inc.

Theoretical Investigation of the Triphosphate Forms of Azidothymidine and Thymidine

“…Our calculations in Table 1 that used full-optimized values with the basis sets (AM1, STO-3G, 3-21G) also gave yields for χ the anti orientation, which agreed with the recent Nuclear Overhauser experimental results Dewar, 1992;Painter et al, 1993;Koch and Popelier, 1995;Painter et al, 2000;Fidanza et al, 2001) . Our χ values were also between the expected experimental (Painter et al, 1993;Painter et al, 2000) values (−95 o to −155 o ), i.e.…”

Theoretical Insights into Three Types of Oxidized Starch-Based Adhesives: Chemical Stability, Water Resistance, and Shearing Viscosity from a Molecular Viewpoint

“…All equilibrium geometries in this paper have been fully optimized at the AM1 semiempirical level without any symmetry restriction [27], which is very favorable to treat the macromolecule. For each optimized stationary point, vibrational frequency analysis obtained at the same theoretical level was performed to determine its character as minima (zero imaginary frequencies).…”

A combined theoretical and experimental study on molecular photonics

“…Nonetheless, we believe that development of models for molecular systems whose calculated spectral-luminescent and photochemical characteristics fit well a set of the corresponding experimental data will help overcome this difficulty. Such models should incorporate not so much a quantum chemical method for calculating the electronic states of the molecule under study as the structural features of the molecular system, clearly defined initial assumptions, an operating sequence and tools to be employed in different stages of the investigation [29,30].…”