Molecular density functional methods

Interestingly, the energy difference is smallest for S7O which as a heterocycle forms a crown-shaped eight-membered ring similar to and isoelec-tronic with the well known 8 structure of 04a symmetry. The transformation of the heterocycle S7O into the homocyclic isomer 7=0 was studied by the molecular dynamics/density functional method but the unrealistically high barrier of 5 eV calculated for this transformation indicates that the system was far from equilibrium during most of the simulation [66]. 

Hpp describes the primary system by a quantum-chemical method. The choice is dictated by the system size and the purpose of the calculation. Two approaches of using a finite computer budget are found If an expensive ab-initio or density functional method is used the number of configurations that can be afforded is limited. Hence, the computationally intensive Hamiltonians are mostly used in geometry optimization (molecular mechanics) problems (see, e. g., [66]). The second approach is to use cheaper and less accurate semi-empirical methods. This is the only choice when many conformations are to be evaluated, i. e., when molecular dynamics or Monte Carlo calculations with meaningful statistical sampling are to be performed. The drawback of semi-empirical methods is that they may be inaccurate to the extent that they produce qualitatively incorrect results, so that their applicability to a given problem has to be established first [67]. 

Rosch, N., Trickey, S. B., 1997, Comment on Concerning die Applicability of Density Functional Methods to Atomic and Molecular Negative Ions [J. Chem. Phys., 105, 862 (1996)] , J. Chem. Phys., 106, 8940. 

Springborg, M., 1997, Some Recent Density-Functional Studies of Molecular Systems , in Density Functional Methods in Chemistry and Materials Science, Springborg, M. (ed.), Wiley, Chichester. 

Zygmunt, S. A., Mueller, R. M., Curtiss, L. A., Iton, L. E., 1998, An Assessment of Density Functional Methods for Studying Molecular Adsorption in Cluster Models of Zeolites , J. Mol. Struct. (Theochem), 430, 9. 

Sola, M., J. Mestres, R. Carbo, and M. Duran. 1996. A Comparative Analysis by Means of Quantum Molecular Similarity Measures of Density Distributions Derived from Conventional ab initio and Density Functional Methods. J. Chem. Phys. 104, 636. 

Topol, I. A., and S. K. Burt. 1993. The calculations of small molecular conformation energy differences by density functional method. Chem. Phys. Lett 204,611. 

The application of theoretical tools for predicting molecular structure, such as ab initio calculations and density functional methods, are discussed in Chapter 6. These tools provide only a first approximation to the molecular structure. There is much room for further development of theoretical molecular structure calculations, but even so such methods have already become a standard part of molecular structure determinations. 

Ab-initio CAChe features all of the above plus ab-initio and density functional methods. This program requires a workstation (Windows NT minimum or SGI and IBM unix-based machines) and can be used to build and visualize results from ab-initio programs (e.g., Gaussian, see description under Gaussian, Inc.). Also, CAChe directly interfaces to Dgauss , a computational chemistry package that uses density functional theory to predict molecular structures, properties, and energetics. 

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