NEXMD v2.0 Software Package for Nonadiabatic Excited State Molecular Dynamics Simulations.

We present NEXMD version 2.0, the second release of the NEXMD (Nonadiabatic EXcited-state Molecular Dynamics) software package. Across a variety of new features, NEXMD v2.0 incorporates new implementations of two hybrid quantum-classical dynamics methods, namely, Ehrenfest dynamics (EHR) and the Ab-Initio Multiple Cloning sampling technique for Multiconfigurational Ehrenfest quantum dynamics (MCE-AIMC or simply AIMC), which are alternative options to the previously implemented trajectory surface hopping (TSH) method. To illustrate these methodologies, we outline a direct comparison of these three hybrid quantum-classical dynamics methods as implemented in the same NEXMD framework, discussing their weaknesses and strengths, using the modeled photodynamics of a polyphenylene ethylene dendrimer building block as a representative example. We also describe the expanded normal-mode analysis and constraints for both the ground and excited states, newly implemented in the NEXMD v2.0 framework, which allow for a deeper analysis of the main vibrational motions involved in vibronic dynamics. Overall, NEXMD v2.0 expands the range of applications of NEXMD to a larger variety of multichromophore organic molecules and photophysical processes involving quantum coherences and persistent couplings between electronic excited states and nuclear velocity.

Libkrylov: A modular open-source software library for extremely large on-the-fly matrix computations.

We present the design and implementation of libkrylov, an open-source library for solving matrix-free eigenvalue, linear, and shifted linear equations using Krylov subspace methods. The primary objectives of libkrylov are flexible API design and modular structure, which enables integration with specialized matrix-vector evaluation "engines." Libkrylov features pluggable preconditioning, orthonormalization, and tunable convergence control. Diagonal (conjugate gradient, CG), Davidson, and Jacobi-Davidson preconditioners are available, along with orthonormal and nonorthonormal (nKs) schemes. All functionality of libkrylov is exposed via Fortran and C application programming interfaces (APIs). We illustrate the performance of libkrylov for eigenvalue calculations arising in time-dependent density functional theory (TDDFT) in the Tamm-Dancoff approximation (TDA) and discuss the convergence behavior as a function of preconditioning and orthonormalization methods.

Electronic Structure of de Novo Peptide ACC-Hex from First Principles.

Proteins are promising components for bioelectronic devices due in part to their biocompatibility, flexibility, and chemical diversity, which enable tuning of material properties. Indeed, an increasingly broad range of conductive protein supramolecular materials have been reported. However, due to their structural and environmental complexity, the electronic structure, and hence conductivity, of protein assemblies is not well-understood. Here we perform an all-atom simulation of the physical and electronic structure of a recently synthesized self-assembled peptide antiparallel coiled-coil hexamer, ACC-Hex. Using classical molecular dynamics and first-principles density functional theory, we examine the interactions of each peptide, containing phenylalanine residues along a hydrophobic core, to form a hexamer structure. We find that while frontier electronic orbitals are composed of phenylalanine, the peptide backbone and remaining residues, including those influenced by solvent, also contribute to the electronic density. Additionally, by studying dimers extracted from the hexamer, we show that structural distortions due to atomic fluctuations significantly impact the electronic structure of the peptide bundle. These results indicate that it is necessary to consider the full atomistic picture when using the electronic structure of supramolecular protein complexes to predict electronic properties.