Timothy J. Dudley

Parallel coupled perturbed CASSCF equations and analytic CASSCF second derivatives.

A parallel algorithm for solving the coupled‐perturbed MCSCF (CPMCSCF) equations and analytic nuclear second derivatives of CASSCF wave functions is presented. A parallel scheme for evaluating derivative integrals and their subsequent use in constructing other derivative quantities is described. The task of solving the CPMCSCF equations is approached using a parallelization scheme that partitions the electronic hessian matrix over all processors as opposed to simple partitioning of the 3 N solution vectors among the processors. The scalability of the current algorithm, up to 128 processors, is demonstrated. Using three test cases, results indicate that the parallelization of derivative integral evaluation through a simple scheme is highly effective regardless of the size of the basis set employed in the CASSCF energy calculation. Parallelization of the construction of the MCSCF electronic hessian during solution of the CPMCSCF equations varies quantitatively depending on the nature of the hessian itself, but is highly scalable in all cases.

Theoretical Investigation of Small Polyatomic Ions Observed in Inductively Coupled Plasma Mass Spectrometry: HxCO+ and HxN2+ (x = 1, 2, 3)

Two series of small polyatomic ions, HxCO+ and HxN2(+) (x = 1, 2, 3), were systematically characterized using three correlated theoretical techniques: density functional theory using the B3LYP functional, spin-restricted second-order perturbation theory, and singles + doubles coupled cluster theory with perturbative triples. On the basis of thermodynamic data, the existence of these ions in inductively coupled plasma mass spectrometry (ICP-MS) experiments is not surprising since the ions are predicted to be considerably more stable than their corresponding dissociation products (by 30-170 kcal/mol). While each pair of isoelectronic ions exhibit very similar thermodynamic and kinetic characteristics, there are significant differences within each series. While the mechanism for dissociation of the larger ions occurs through hydrogen abstraction, the triatomic ions (HCO+ and HN2(+)) appear to dissociate by proton abstraction. These differing mechanisms help to explain large differences in the abundances of HN2(+) and HCO+ observed in ICP-MS experiments.

Theoretical study of the formation and isomerization of Al2H2

The lowest singlet and triplet potential energy surfaces of Al2H2 have been characterized using a full-valence complete active space self-consistent field (CASSCF) wavefunction. The CASSCF geometries of minima on the singlet potential energy surface are compared to those previously reported using density functional theory (DFT) and coupled cluster (CCSD(T)) methods. Energies at the CASSCF geometries are corrected for dynamic correlation effects using multi-reference second-order perturbation theory (MRMP2) and CCSD(T). Relative energies calculated at the MRMP2//CASSCF level are comparable to those evaluated at CCSD(T) optimized geometries. This approach to characterizing stationary points and calculating relative energies is utilized to describe isomerization pathways between minima on the lowest singlet and triplet surfaces and to characterize pathways leading to formation of bound Al2H2 species from various fragments (e.g., AlH, AlH2, Al2, and H2). The results presented here confirm that the global minimum is the singlet dibridged isomer, with other singlet isomers lying slightly higher in energy. Though not previously analysed, most triplet structures were found to be less than 20 kcal mol−1 higher in energy than their singlet counterparts. A purely attractive singlet reaction channel, involving the insertion of H2 directly into the Al–Al bond of an excited Al2 species, was located and found to be exothermic by about 40 kcal mol−1. Based on energy and frequency analyses of the singlet and triplet surfaces, previous conclusions that only the singlet dibridged and monobridged isomers have been observed in matrix isolation experiments are analysed further.

Theoretical Investigation of the Gas-Phase Reactions of CrO+ with Ethylene

The potential energy surfaces associated with the reactions of chromium oxide cation (CrO(+)) with ethylene have been characterized using density functional, coupled-cluster, and multireference methods. Our calculations show that the most probable reaction involves the formation of acetaldehyde and Cr(+) via a hydride transfer involving the metal center. Our calculations support previous experimental hypotheses that a four-membered ring intermediate plays an important role in the reactivity of the system. We have also characterized a number of viable reaction pathways that lead to other products, including ethylene oxide. Due to the experimental observation that CrO(+) can activate carbon-carbon bonds, a reaction pathway involving C-C bond cleavage has also been characterized. Since many of the reactions involve a change in the spin state in going from reactants to products, locations of these spin surface crossings are presented and discussed. The applicability of methods based on Hartree-Fock orbitals is also discussed.

Scalable correlated electronic structure theory

The approach taken in Ames to advance high-level electronic structure theory has been a combination of the development and implementation of new and novel methods with the continuing development of strategies to optimize scalable computing. This work summarizes advances on both fronts. Several new methods have been implemented under the Distributed Data Interface (DDI), most recently including analytic Hessians for both Hatree- Fock and CASSCF (complete active space self-consistent field) wavefunctions, gradients for restricted open shell second order perturbation theory, and the fragment molecular orbital method (FMO). Exciting new method developments include the FMO method and the CEEIS (Correlation Energy Extrapolation by Intrinsic Scaling) method for efficiently approaching the exact energy for atomic and molecular systems.

Conformational analysis via calculations and NMR spectroscopy for isomers of the mono(imino)pyridine ligand, 2-{(2,6-Me2-C6H3)NC(i-Pr)}C5H4N

Sterically hindered (imino)pyridine 2-{(2,6-Me2-C6H3)NC(i-Pr)}C5H4N (1) was synthesized via addition of isolated imidoyl chloride to an in situ lithiated pyridine. Room temperature 1-D and 2-D NMR spectroscopy reveals two rapidly equilibrating isomers in solution. Interconversion of these two isomers was verified by 2D-EXSY NMR spectroscopy. Calculations at the B3LYP and MP2 levels of theory reveal four relevant isomers, with two atropisomers of E geometry (1-EA and 1-EB) and two atropisomers of Z geometry (1-ZA and 1-ZB). A simple carbon–carbon bond rotation to alter the orientation of the isopropyl group provides a fifth, related conformer, 1-ZB′, that is the most stable species at the MP2 level. The transition states for E/Z isomerization and the isomerization pathways between atropisomers have been characterized. Comparison of experimental and ab initio NMR chemical shifts in combination with NOE analysis suggests that isomers 1-EB and 1-ZB/1-ZB′ are the dominant species in our solution phase NMR studies. Our understanding of the isomerization behavior of 1 will help inform the future design of readily complexed, sterically hindered mono(imine) and bis(imine) ligands.

Theoretical Investigation of the Gas-Phase Reaction of CrO+ with Propane

Transition metal oxide cations (e.g., MO+) have been shown to oxidize small alkanes in the gas phase. The chromium oxide cation is of particular interest because it is more reactive than oxides of earlier transition metals but is more selective than oxides of later transition metals. The reaction of CrO+ with propane has been shown to produce a number of products: propanol, propene, ethene, and hydrogen. Few theoretical studies exist for reactions of simple transition metal oxide cations with larger alkanes. We have analyzed the potential energy surfaces associated with the reaction of CrO+ with propane using two DFT methods, B3LYP and M06-L. Energetically viable reaction paths leading to each experimentally observed product have been characterized. Each reaction path begins with formation of a reactive intermediate in which either an α- or β-hydrogen from propane is extracted by the oxygen atom of CrO+. While pathways leading to formation of hydrogen and ethene were found to occur on a single spin surface, energetically viable pathways to forming propanol and propene require a transition from the quartet spin surface to the sextet surface. The minimum-energy crossing points between the quartet and sextet surfaces were found to be well below the energy level of the reactants and structurally resemble the initial reactive intermediates.

Generalized Van Vleck perturbation theory study of chlorine monoxide

The second order generalized Van Vleck perturbation (GVVPT2) variant of multireference perturbation theory for molecular electronic structure theory was used to study the ground and first excited states of the atmospherically important molecule chlorine monoxide (ClO). Unlike multiconfigurational self-consistent field (MCSCF) results, GVVPT2 shows that the A 2Π state has a local minimum, in agreement with experiment. This study supports the growing evidence that GVVPT2 can be used to study complex electronic structures, including those involving excited states and radicals.