Ida Nielsen

Cobalt−Porphyrin Catalyzed Electrochemical Reduction of Carbon Dioxide in Water. 2. Mechanism from First Principles

We apply first principles computational techniques to analyze the two-electron, multistep, electrochemical reduction of CO(2) to CO in water using cobalt porphyrin as a catalyst. Density functional theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO(2) bound to cobalt. By use of potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH < 9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized.

Parallel direct implementations of second‐order perturbation theories

Two algorithms are presented for parallel direct computation of energies with second‐order perturbation theory. Closed‐shell MP2 theory as well as the open‐shell perturbation theories OPT2(2) and ZAPT2 have been implemented. The algorithms are designed for distributed memory parallel computers. The first algorithm exhibits an excellent load balance and scales well when relatively few processors are used, but a large communication overhead reduces the efficiency for larger numbers of processors. The other algorithm employs very little interprocessor communication and scales well for large systems. In both implementations the memory requirement has been reduced by allowing the two‐electron integral transformation to be performed in multiple passes and by distributing the (partially) transformed integrals between processors. Results are presented for systems with up to 327 basis functions.

Toward resolution of the silicon dicarbide (SiC2) saga: Ab initio excursions in the web of polytopism

The long-standing problem of the topography, energetics, and vibrational dynamics of the ground-state surface of SiC2 is systematically investigated by means of the gamut of state-of-the-art electronic structure methods, including single-reference correlation techniques as extensive as the coupled-cluster singles and doubles method augmented by a perturbative triples term [CCSD(T)], the Brueckner doubles method (BD) with analogous contributions from both triple and quadruple excitations [BD(TQ)], and second-through fifth-order Mo/ller–Plesset perturbation theory (MP2–MP5), as well as the multiconfigurational complete-active-space self-consistent-field [CASSCF(12,12)] approach. The one-particle basis sets for these studies ranged from Si[6s4p1d], C[4s2p1d] to Si[7s6p4d3f2g1h], C[6s5p4d3f2g1h]. The methodological analysis resolves the polytopism problem regarding the mercurial potential energy surface for the circumnavigation of Si+ about C2− in silicon dicarbide, whose topography is shown to exhibit almost...

A new direct MP2 gradient algorithm with implementation on a massively parallel computer

Abstract A new direct second-order Moller-Plesset gradient algorithm is presented. By avoiding generation of molecular orbital integrals with three virtual indices, the memory requirement for a calculation with n basis functions and o occupied orbitals is reduced to on 2 , and overall computational savings are obtained. The algorithm has been implemented with distributed data on a massively parallel computer, the Intel Paragon. The parallelization scheme provides high parallel efficiency, and the performance of the algorithm is illustrated for calculations running on up to 64 processors.

Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 1. A density functional study of intermediates.

The reduction of carbon dioxide by cobalt porphyrins is thought to be a multistep reaction with several possible intermediates and reaction pathways. We here investigate a number of possible intermediates in this reaction using density functional theory, including both hybrid (B3LYP) and pure (PBE and BP86) functionals. Optimum structures are located, and harmonic vibrational frequencies and thermal corrections are computed for the low-lying electronic states for all intermediates. Free energies of solvation are predicted for all species, providing a reaction profile in the aqueous phase, which enables identification of likely pathways. Finally, the reaction energy for the binding of carbon dioxide to the cobalt porphine cation is determined in the gas phase and in solution.

Local Møller-Plesset Perturbation Theory: A Massively Parallel Algorithm.

A massively parallel algorithm is presented for computation of energies with local second-order Møller-Plesset (LMP2) perturbation theory. Both the storage requirement and the computational time scale linearly with the molecular size. The parallel algorithm is designed to be scalable, employing a distributed data scheme for the two-electron integrals, avoiding communication bottlenecks, and distributing tasks in all computationally significant steps. A sparse data representation and a set of generalized contraction routines have been developed to allow efficient massively parallel implementation using distributed sparse multidimensional arrays. High parallel efficiency of the algorithm is demonstrated for applications employing up to 100 processors.

Component architectures for quantum chemistry: forging new capabilities and insights

We review the use of the Common Component Architecture approach within the quantum chemistry domain to tackle the software engineering challenges which arise as advanced algorithms are adopted and growing numbers of software packages are integrated to study complex, coupled physical phenomena. The development of common interfaces has allowed the adoption of advanced optimization solvers and high-level interchangeability of quantum chemistry packages. Components have been created which manage multiple levels of parallelism, providing much more efficient usage of parallel machines. Early efforts towards low-level integration of chemistry packages are examined. The ability to share intermediate data expands the capabilities available to any one software package, thereby enabling the rapid development of advanced methods. New methods for the study of reactions involving heavy elements, which depend on our component environment, are highlighted.

Scalable Fault Tolerant Algorithms for Linear-Scaling Coupled-Cluster Electronic Structure Methods

By means of coupled-cluster theory, molecular properties can be computed with an accuracy often exceeding that of experiment. The high-degree polynomial scaling of the coupled-cluster method, however, remains a major obstacle in the accurate theoretical treatment of mainstream chemical problems, despite tremendous progress in computer architectures. Although it has long been recognized that this super-linear scaling is non-physical, the development of efficient reduced-scaling algorithms for massively parallel computers has not been realized. We here present a locally correlated, reduced-scaling, massively parallel coupled-cluster algorithm. A sparse data representation for handling distributed, sparse multidimensional arrays has been implemented along with a set of generalized contraction routines capable of handling such arrays. The parallel implementation entails a coarse-grained parallelization, reducing interprocessor communication and distributing the largest data arrays but replicating as many arrays as possible without introducing memory bottlenecks. The performance of the algorithm is illustrated by several series of runs for glycine chains using a Linux cluster with an InfiniBand interconnect.