Mark C. Palenik

Model of the Oxygen Evolving Complex Which is Highly Predisposed to O–O Bond Formation.

Light-driven water oxidation is a fundamental reaction in the biosphere. The Mn4Ca cluster of photosystem II cycles through five redox states termed S0-S4, after which oxygen is evolved. Critically, the timing of O-O bond formation within the Kok cycle remains unknown. By combining recent crystallographic, spectroscopic, and DFT results, we demonstrate an atomistic S3 state model with the possibility of a low barrier to O-O bond formation prior to the final oxidation step. Furthermore, the associated one electron oxidized S4 state does not provide more advantages in terms of spin alignment or the energy of O-O bond formation. We propose that a high energy peroxide isoform of the S3 state can preferentially be oxidized by Tyr zox in the course of final electron transfer leading to O2 evolution. Such a mechanism may explain the peculiar kinetic behavior of O2 evolution as well as serve as an evolutionary adaptation to avoid release of the harmful peroxides.

Variationally fitting the total electron-electron interaction

Density fitting is used throughout quantum chemistry to simplify the electron-electron interaction energy (EE). A fundamental property of quantum chemistry, and DFT in particular, is that a variational principle connects the EE to a potential. Density fitting generally does not preserve this connection. Herein, we describe the construction of a robust EE that is variationally connected to fitted potentials in all electronic structure methods. For DFT, this results in new fitting equations which are satisfied at an energy saddle point in multidimensional fitting space.

Density perturbation theory.

Despite the fundamental importance of electron density in density functional theory, perturbations are still usually dealt with using Hartree-Fock-like orbital equations known as coupled-perturbed Kohn-Sham (CPKS). As an alternative, we develop a perturbation theory that solves for the perturbed density directly, removing the need for CPKS. This replaces CPKS with a true Hohenberg-Kohn density perturbation theory. In CPKS, the perturbed density is found in the basis of products of occupied and virtual orbitals, which becomes ever more over-complete as the size of the orbital basis set increases. In our method, the perturbation to the density is expanded in terms of a series of density basis functions and found directly. It is possible to solve for the density in such a way that it makes the total energy stationary even if the density basis is incomplete.

Degenerate density perturbation theory

Fractional occupation numbers can be used in density functional theory to create a symmetric Kohn-Sham potential, resulting in orbitals with degenerate eigenvalues. We develop the corresponding perturbation theory and apply it to a system of

Hydrogen-bonded intermediates and transition states during spontaneous and acid-catalyzed hydrolysis of the carcinogen (+)-anti-BPDE.

N_d

Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

degenerate electrons in a harmonic oscillator potential. The order-by-order expansions of both the fractional occupation numbers and unitary transformations within the degenerate subspace are determined by the requirement that a differentiable map exists connecting the initial and perturbed states. Using the X

X-ray Emission Spectroscopy of Mn Coordination Complexes Toward Interpreting the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II

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Rapid Evolution of the Photosystem II Electronic Structure during Water Splitting

exchange-correlation (XC) functional, we find an analytic solution for the first-order density and first through third-order energies as a function of

General degeneracy in density functional perturbation theory

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Energy Continuity in Degenerate Density Functional Perturbation Theory

, with and without a self-interaction correction. The fact that the XC Hessian is not positive definite plays an important role in the behavior of the occupation numbers.