General Internal Medicine

We investigate gas-phase neutral and cationic hydrides formed by 3d transition metals from Sc to Cu with density functional theory ͑DFT͒ methods. The performance of two exchange-correlation functionals, Boese-Martin for kinetics ͑BMK͒ and Tao-Perdew-Staroverov-Scuseria ͑TPSS͒, in predicting bond lengths and energetics, electronic structures, dipole moments, and ionization potentials is evaluated in comparison with available experimental data. To ensure a unique self-consistent field ͑SCF͒ solution, we use stability analysis, Fermi smearing, and continuity analysis of the potential energy curves. Broken-symmetry approach was adapted in order to get the qualitatively correct description of the bond dissociation. We found that on average BMK predicted values of dissociation energies and ionization potentials are closer to experiment than those obtained with high level wave function theory methods. This agreement deteriorates quickly when the fraction of the Hartree-Fock exchange in DFT functional is decreased. Natural bond orbital ͑NBO͒ population analysis was used to describe the details of chemical bonding in the systems studied. The multireference character in the wave function description of the hydrides is reproduced in broken-symmetry DFT description, as evidenced by NBO analysis. We also propose a new scheme to correct for spin contamination arising in broken-symmetry DFT approach. Unlike conventional schemes, our spin correction is introduced for each spin-polarized electron pair individually and therefore is expected to yield more accurate energy values. We derive an expression to extract the energy of the pure singlet state from the energy of the broken-symmetry DFT description of the low spin state and the energies of the high spin states ͑pentuplet and two spin-contaminated triplets in the case of two spin-polarized electron pairs͒. The high spin states are build with canonical natural orbitals and do not require SCF convergence.

The stable geometries and atomization energies for the clusters Ni n (n=2-5) are predicted with all-electron density functional theory (DFT), using the BMK hybrid functional and a Gaussian basis set. Possible isomers and several spin states of these nickel clusters are considered systematically. The ground spin state and the lowest energy isomers are identified for each cluster size. The results are compared to available experimental and other theoretical data. The molecular orbitals of the largest cluster are plotted for all spin states. The relative stabilities of these states are interpreted in terms of superatom orbitals and no-pair bonding.

A clear advantage of broken symmetry (BS) unrestricted density functional theory DFT is qualitatively correct description of bond dissociation process, but its disadvantage is that spin-polarized Slater determinant is no longer a pure spin state (a.k.a. spin contamination). We propose a new approach to eliminate the spin-contamination, based on canonical Natural Orbitals (NO). We derive an expression to extract the energy of the pure singlet state given in terms of energy of BS DFT solution, the occupation number of the bonding NO, and the energy of the higher state built on these bonding and antibonding NOs (as opposed to self-consistent Kohn-Sham orbitals). Thus, unlike spincontamination correction schemes by Noodleman and Yamaguchi, spincorrection is introduced for each correlated electron pair individually and thus expected to give more accurate results. We validate this approach on two examples, a simple diatomic H 2 and transition metal hydride MnH.

Nanoelectronics and photonics applications of single wall carbon nanotubes (SWNT) are feasible only if SWNTs have specific chirality. The knowledge of the detailed mechanism for SWNT synthesis would allow one to optimize the chemical vapor deposition (CVD) process and may help to gain control over selectivity of SWNT synthesis. While it is not probably feasible to study this mechanism experimentally, it could be analyzed using molecular simulations. Here we propose multiscale computer modeling of CVD process. High theory level can be used for di-and tri-atomic fragments, in order to generate parameters for bond order force field. In turn, force field simulations will be used to characterize the chemical origin and thermochemical properties of the intermediates and transition states. This will allow predicting the rate constants for the elementary steps, which are then used in kinetic Monte Carlo simulations to describe SWNT growth at realistic time scales.

The computational description of the catalytic processes on the surface of transition metals (TMs) requires methods capable of accurate prediction of the bond forming and breaking between the atoms of metal and other elements. In our previous report [Goel and Masunov, J Chem Phys, 129, 214302, 2008], we studied TM hydrides and found that Boese-Martin functional for kinetics (BMK) combined with broken symmetry approach described dissociation process more accurately than multireference wavefunction theory (WFT) methods and some other functionals. Here, we investigate the binding energy, geometry, electronic structure, and potential energy curves for diatomic TM carbides using several exchange-correlation functionals. The functionals that include explicit dependence on the kinetic energy density (s-functionals) are considered, among others. We have found M05-2x performance to be the best, followed by BMK, when compared with experimental and high level WFT energetics. This agreement deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange is decreased. Scalar relativistic corrections yield mixed results for bond lengths and bond energies. The natural bond orbital analysis provides useful insight in description of stable spin state over others in these diatomics.