Stephen T. Elbert

Chemical binding and electron correlation in diatomic molecules as described by the FORS model and the FORS-IACC model

Using the model of the Full Optimized Reaction Space including the Intra-Atomic Correlation Correction, binding energies and other electronic properties have been calculated for several states of a number of diatomic molecules. In most cases this theoretical approach yields results agreeing with experimental values to within 0.2 eV. The investigation covers the molecules BH, CH, NH, OH, FH, N2, O2, F2.

The ring opening of cyclopropylidene to allene: key features of the accurate reaction surface

SummaryThe global potential energy surface, determined in the first paper [1] for the groundstate ring opening of cyclopropylidene to allene, is complemented by accurate calculations of its key regions. The basis set is extended and polarization functions are included. The full configuration space of four electrons in four reactive orbitals is enlarged to the full configuration space of eight electrons in eight active orbitals by including correlations in the unbroken, but stretched CC sigma bonds. The effect of further single and double excitations is examined. The geometries and relative energies of the critical regions are found to change only little except for the ring-opening energy barrier which is lowered to about 7 kcal/mol, in good agreement with the experimental estimate of about 6 kcal/mol. Furthermore, the bifurcation is shown to occurafter the transition state, in the neighborhood of a conical intersection on the steepest descent path from the ring-opening transition state to the allene isomerization transition state. The steepest descent paths and the conical intersection are documented in detail. The cogwheel-like free internal rotation of the two methyl groups is confirmed by the accurate calculations. A similar richness of features is believed to exist on many potential energy surfaces governing chemical reactions.

Extracting more than a few eigenvectors from a dense real symmetric matrix: Optimal algorithms versus the architectural constraints of the FPS-X64

Ten widely available sets of routines, including HQRII, QCPE GIVENS and EISPACK 3, were evaluated for reliability, robustness, accuracy, speed, compactness, portability and simplicity. All were found lacking in one or more areas. Modified versions of the EISPACK routines TRED3, TQLRAT, TINVIT and TRBAK3 performed somewhat better. Changes to TINVIT were especially important for improved speed, accuracy and reliability. To achieve the maximum capabilities of the FPS-X64 series of computers access to table memory is required, but since the FORTRAN compiler does not allow this and there is no library support for the required operations, it was necessary to write three routines in APAL. The standard algorithm needs to be modified before full efficiency can be achieved for the back transformation.

Trends in Homolytic Bond Dissociation Energies of Five- and Six-Coordinate Hydrides of Group 9 Transition Metals: Co, Rh, Ir

The homolytic bond dissociation energies of a series of five- and six-coordinate mono- and dihydride complexes of the type HM(diphosphine)2 and [H2M(diphosphine)2]+ (where M = Co, Rh, and Ir) are calculated and compared with experimental values. This work probes the relationship between the homolytic bond dissociation energies (HMBDEs) of these complexes in these two different coordination environments and formal oxidation states. The results of these calculations and previous experimental observations suggest that for M = Rh the HMBDE of the five-coordinate HM(diphosphine)2 species are 0-2 kcal/mol larger than the HMBDE of the corresponding six-coordinate [H2M(diphosphine)2]+ species. For M = Ir the bond energies of the five- and six-coordinate complexes are nearly the same and for M = Co the six-coordinate species are 1-5 kcal/mol less than the corresponding five-coordinate species. Simplified models of large and complicated ligands seem to capture the essential trends and give very good estimates of these thermodynamic properties compared with experimentally available data that are difficult to obtain.