Efficiency Limits of Energy Conversion by Light-Driven Redox Chains.

The conversion of absorbed sunlight to spatially separated electron-hole pairs is a crucial outcome of natural photosynthesis. Many organisms achieve near-unit quantum yields of charge separation (one electron-hole pair per incident photon) by dissipating as heat more than half of the light energy that is deposited in the primary donor. Might alternative choices have been made by Nature that would sacrifice quantum yield in favor of producing higher energy electron/hole pairs? Here, we use a multisite electron hopping model to address the kinetic and thermodynamic compromises that can be made in electron transfer chains, with the aim of understanding Nature's choices and opportunities in bioinspired energy-converting systems.

Undulating Free Energy Landscapes Buffer Redox Chains from Environmental Fluctuations.

Roller-coaster or undulating free energy landscapes, with alternating high and low potential cofactors, occur frequently in biological redox chains. Yet, there is little understanding of the possible advantages created by these landscapes. We examined the tetraheme subunit associated with reaction centers, comparing the dynamics of the native protein and of hypothetical (in silico) mutants.

Mapping Simulated Two-Dimensional Spectra to Molecular Models Using Machine Learning.

Two-dimensional (2D) spectroscopy encodes molecular properties and dynamics into expansive spectral data sets. Translating these data into meaningful chemical insights is challenging because of the many ways chemical properties can influence the spectra. To address the task of extracting chemical information from 2D spectroscopy, we study the capacity of simple feedforward neural networks (NNs) to map simulated 2D electronic spectra to underlying physical Hamiltonians.

Semiglobal diabatic potential energy matrix for the N-H photodissociation of methylamine.

We constructed an analytic diabatic potential energy matrix (DPEM) that describes the N-H photodissociation of methylamine; the electronic state space includes the ground and first excited singlet states. The input for the fit was calculated by extended multi-state complete active space second-order perturbation theory. The data were diabatized using the dipole-quadrupole diabatization method in which we incorporated a coordinate-dependent weighting scheme for the contribution of the quadrupole moments.

Conservation of Angular Momentum in Direct Nonadiabatic Dynamics.

Direct nonadiabatic dynamics is used to study processes involving multiple electronic states from small molecules to materials. Compared with dynamics with fitted analytical potential energy surfaces, direct dynamics is more user-friendly in that it obtains all needed energies, gradients, and nonadiabatic couplings (NACs) by electronic structure calculations. However, the NAC that is usually used does not conserve angular momentum or the center of mass in widely used mixed quantum-classical nonadiabatic dynamics algorithms, in particular, trajectory surface hopping, semiclassical Ehrenfest, and coherent switching with decay of mixing.

Extended Hamiltonian molecular dynamics: semiclassical trajectories with improved maintenance of zero point energy.

It is well known that classical trajectories, even if they are initiated with zero point energy (ZPE) in each mode (trajectories initiated this way are commonly called quasiclassical trajectories), do not maintain ZPE in the final states. The energy of high-frequency modes will typically leak into low-frequency modes or relative translation of subsystems during the time evolution. This can lead to severe problems such as unphysical dissociation of a molecule, production of energetically disallowed reaction products, and unphysical product energy distributions.

Direct diabatization based on nonadiabatic couplings: the N/D method.

Diabatization converts adiabatic electronic states to diabatic states, which can be fit with smooth functions, thereby decreasing the computational time for simulations. Here we present a new diabatization scheme based on components of the nonadiabatic couplings and the adiabatic energy gradients. The nonadiabatic couplings are multi-dimensional vectors that are singular along conical intersection seams, and this makes them essentially impossible to fit; furthermore they have unphysical aspects due to the assumptions of the generalized Born-Oppenheimer scheme, and therefore they are not usually used in diabatization schemes.

Dual-Functional Tamm-Dancoff Approximation with Self-Interaction-Free Orbitals: Vertical Excitation Energies and Potential Energy Surfaces near an Intersection Seam.

Recently we have developed the dual-functional Tamm-Dancoff approximation (DF-TDA) method. DF-TDA is an alternative to linear-response time-dependent density functional theory (LR-TDDFT) with the advantage of providing a correct double-cone topology of S/S conical intersections. In the DF-TDA method, we employ different functionals, which are denoted G and F, for orbital optimization and Hamiltonian construction.

Dual-Functional Tamm-Dancoff Approximation: A Convenient Density Functional Method that Correctly Describes S/S Conical Intersections.

Time-dependent Kohn-Sham density functional theory has been used successfully to compute vertical excitation energies, especially for large molecular systems. However, the lack of double excitation character in the excited amplitudes produced by linear response in the adiabatic approximation holds it back from broader applications in photochemistry; for example, it shows (3N - 7)-dimensional conical intersection seams (where N is the number of atoms) between ground and excited states, although the correct dimensionality is 3N - 8. In this letter, we present a new, conceptually simple, easy-to-implement, and easy-to-use way to employ time-dependent Kohn-Sham density functional theory that has global accuracy comparable with the conventional single-functional version and that recovers the double cone topology of the potential energy surfaces at S/S conical intersection seams.