Ab Initio Multiple Spawning Dynamics Research Articles

SSAIMS-Stochastic-Selection Ab Initio Multiple Spawning for Efficient Nonadiabatic Molecular Dynamics.

Ab initio multiple spawning provides a powerful and accurate way of describing the excited-state dynamics of molecular systems, whose strength resides in the proper description of coherence effects during nonadiabatic processes thanks to the coupling of trajectory basis functions. However, the simultaneous propagation of a large number of trajectory basis functions can be numerically inconvenient. We propose here an elegant and simple solution to this issue, which consists of (i) detecting uncoupled groups of coupled trajectory basis functions and (ii) selecting stochastically one of these groups to continue the ab initio multiple spawning dynamics. We show that this procedure can reproduce the results of full ab initio multiple spawning dynamics in cases where the uncoupled groups of trajectory basis functions stay uncoupled throughout the dynamics (which is often the case in high-dimensional problems). We present and discuss the aforementioned idea in detail and provide simple numerical applications on indole, ethylene, and protonated formaldimine, highlighting the potential of stochastic-selection ab initio multiple spawning.

Nonlinear dimensionality reduction for nonadiabatic dynamics: The influence of conical intersection topography on population transfer rates

Conical intersections play a critical role in the nonadiabatic relaxation of excited electronic states. However, there are an infinite number of these intersections and it is difficult to predict which are actually relevant. Furthermore, traditional descriptors such as intrinsic reaction coordinates and steepest descent paths often fail to adequately characterize excited state reactions due to their highly nonequilibrium nature. To address these deficiencies in the characterization of excited state mechanisms, we apply a nonlinear dimensionality reduction scheme (diffusion mapping) to generate reaction coordinates directly from ab initio multiple spawning dynamics calculations. As illustrated with various examples of photoisomerization dynamics, excited state reaction pathways can be derived directly from simulation data without any a priori specification of relevant coordinates. Furthermore, diffusion maps also reveal the influence of intersection topography on the efficiency of electronic population transfer, providing further evidence that peaked intersections promote nonadiabatic transitions more effectively than sloped intersections. Our results demonstrate the usefulness of nonlinear dimensionality reduction techniques as powerful tools for elucidating reaction mechanisms beyond the statistical description of processes on ground state potential energy surfaces.

Protonic Gating of Excited-State Twisting and Charge Localization in GFP Chromophores: A Mechanistic Hypothesis for Reversible Photoswitching

Reversible photoswitching fluorescent proteins can be photoswitched between fluorescent and nonfluorescent states by different irradiation regimes. Accumulating spectroscopic and crystallographic evidence suggest a correlated change in protonation state and methine bridge isomerism of the chromophore. The anion can decay by photoisomerization of either of the methine bonds, but only one channel can act as a switch. Using ab initio multiple spawning dynamics simulations, we show that protonation is sufficient to change the photoisomerization channel in the chromophore. We propose that this behavior can underlie a switch given certain other conditions. We also propose a basis for coupling between excited-state basicity changes and selection of the photoisomerization channel based on the polarity of twisted charge-transfer states for neutral and anionic forms of the chromophore.

Ab Initio Multiple Spawning Dynamics of Excited Butadiene: Role of Charge Transfer

Ab initio multiple spawning simulations of the photochemical reaction dynamics of s-trans-1,3-butadiene were performed. It is found that nonadiabatic events involving two low-lying excited states begin as early as 10 fs after excitation, resulting in the population being split between the bright 1(1)B(u) state and the dark 2(1)A(g) state. The molecule subsequently twists about a terminal carbon-carbon bond regardless of whether it is on the 1(1)B(u) or 2(1)A(g) electronic state. This twisting motion leads to conical intersections between S(1) and S(0). Several regions of the intersection seam involving states of differing character are accessed. The regions of the seam involving intersection between a state of charge-transfer character and a state of covalent character dominate the quenching dynamics, but intersections between two covalent states are also accessed a small percentage of the time. The existence and relative energies of these intersections are validated by optimization at the multistate complete active space second-order perturbation level of theory (MS-CASPT2). Our results point to a new mechanism for photoisomerization of butadiene that emphasizes the role of charge-transfer states.

Electrostatic control of photoisomerization in the photoactive yellow protein chromophore: Ab initio multiple spawning dynamics

The excited state dynamics of an anionic model photoactive yellow protein (PYP) chromophore have been investigated using the ab initio multiple spawning (AIMS) method. We observe two isomerization products, differing in the bond about which isomerization takes place. We also carried out AIMS simulations including a single point charge to mimic the Arg52 residue which is present in the protein environment. We show that a single point charge is sufficient to change the photoproduct distribution such that isomerization is restricted to only one of the two bonds where isomerization is observed in the isolated chromophore. These results corroborate recent speculations concerning the role of Arg52 in the photocycle of PYP.

Ab initio/interpolated quantum dynamics on coupled electronic states with full configuration interaction wave functions

We present a new approach to first-principles molecular dynamics that combines a general and flexible interpolation method with ab initio evaluation of the potential energy surface. This hybrid approach extends significantly the domain of applicability of ab initio molecular dynamics. Use of interpolation significantly reduces the computational effort associated with the dynamics over most of the time scale of interest, while regions where potential energy surfaces are difficult to interpolate, for example near conical intersections, are treated by direct solution of the electronic Schrödinger equation during the dynamics. We demonstrate the concept through application to the nonadiabatic dynamics of collisional electronic quenching of Li(2p). Full configuration interaction is used to describe the wave functions of the ground and excited electronic states. The hybrid approach agrees well with full ab initio multiple spawning dynamics, while being more than an order of magnitude faster.