Fragmentation Methods: A Route to Accurate Calculations on Large Systems Mark S. Gordon,* Dmitri G. Fedorov, Spencer R. Pruitt, and Lyudmila V. Slipchenko Department of Chemistry and Ames Laboratory, Iowa State University, Ames Iowa 50011, United States Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

Fragmentation methods: a route to accurate calculations on large systems.

Three exciting new methods that address the accurate prediction of processes and properties of large molecular systems are discussed. The systematic fragmentation method (SFM) and the fragment mole...

Accurate methods for large molecular systems.

The ultraviolet photoemission and optical absorption spectra of pyrrole and polypyrrole are analyzed using the spectroscopically parametrized CNDO/S3 model. Calculated charge densities, bond orders, and orbital eigenvalues for pyrrole are compared with the extensive literature on this molecule. The densities of valence states are computed for pyrrole, oligomers of pyrrole, and an infinite planar polypyrrole chain. They provide a good description of ultraviolet valence electron photoemission spectra measured for gas‐phase pyrrole, condensed thin films of pyrrole, and thin‐film polypyrrole. The photogenerated radical cation states in polypyrrole seem to extend over four or more pyrrole units for the higher binding energy σ‐electron states but are localized on fewer units for the lowest binding energy π‐electron states. The relationship between the density of valence states and the energy band structure of polypyrrole is established. The energies of the first few dipole allowed optical absorption transitions...

Electronic structure of polypyrrole and oligomers of pyrrole

An additive procedure (SIBFA) is developed for the rapid computation of conformational energy variations in very large molecules. The macromolecule is built out of constitutive molecular fragments and the intramolecular energy is computed as a sum of interaction energies between the fragments. The electrostatic and the polarization components are calculated using multicenter multipole expansions of theab initio SCF electron density of the fragments. The repulsion component is obtained as a sum of bond and lone pair interactions. Tests of the procedure on a series of model compounds containing ether oxygens and pyridine-like nitrogens are reported and compared with the results of correspondingab initio SCF calculations. The resulting methodology is compatible with the simultaneous computation of intermolecular interactions.

Theoretical studies of molecular conformation. Derivation of an additive procedure for the computation of intramolecular interaction energies. Comparison with ab initio SCF computations

The electronic structure of molecules by a many-body approach: II. Ionization potentials one-electron properties of pyridine and phosphoridine

An ab initio procedure for investigation of large molecules is described. The procedure involves two steps, the first of which is the determination of a set of orbitals by the examination of molecular fragments. These molecular fragments and associated orbitals are then combined to form large molecules of interest, using an ordinary LCAO MO SCF‐type procedure. The molecular fragment investigations employed are designed to obtain nonlinear parameters for use in large molecules, and are not used in this study to obtain “small molecules” from which “large molecules” can be formed. Several descriptions of molecular fragments are explored, in which floating spherical Gaussian orbitals are employed, and applications of the method to ethane, propane, ethylene, and benzene are given. Comparisons are made with other methods, and several general characteristics of the method are also discussed.

Ab Initio Calculations on Large Molecules Using Molecular Fragments. Hydrocarbon Characterizations

Ab initio calculations on large molecules using molecular fragments. Electronic and geometric characterization of acetylcholine

Ab initio calculations on large molecules using molecular fragments. Characterization of nitrogen-containing molecules

A number of first order electronic properties for the hydrocarbons C2H6, C3H8, C2H4, C6H6, and C10H8 are investigated. The wavefunctions employed here result from SCF calculations, using basis orbitals that have been optimized in molecular fragment studies. Comparisons are made with experimental values as well as with other calculated values, and the suitability of various molecular fragment bases is discussed.

Donald W. Genson

Papers

Ab Initio Calculations on Large Molecules Using Molecular Fragments. Hydrocarbon Characterizations

Ab initio calculations on large molecules using molecular fragments. Electronic and geometric characterization of acetylcholine

Ab initio calculations on large molecules using molecular fragments. Characterization of nitrogen-containing molecules

Ab initio calculations on large molecules using molecular fragments First order electronic properties for hydrocarbons

Ab inito-berechnungen grosser molekuele ueber molekuelfragmente, elektronische und geometrische charakterisierung von acetylcholin