In collaboration with the group of Prof. Petr Slavíček in Prague, we investigated the role of initial conditions when simulating the excited-state dynamics of molecules. This question is particularly relevant if one is interested in the simulation of photoexcitation process triggered by long pulses, i.e., when the formation of a nuclear wavepacket in the excited state is not guaranteed. More information can be found in our article just accepted in Faraday Discussions.
In the first article, we studied with Dr. Federica Agostini the dynamics of a nuclear wavepacket through a conical intersection using the formalism of the Exact Factorization (EF). In a previous work, we looked at a model of the photoisomerization of retinal for this purpose. In this new article, we employed a different model for the potential energy surfaces and played with their diabatic coupling to study how the time-dependent potential energy surface and vector potential – key quantities of the EF – behave in different nonadiabatic regimes.
In a second article, we studied the excited-state dynamics of oxirane using the method coined coupled-trajectory mixed quantum/classical (CT-MQC), combined with linear-response time-dependent density functional theory (LR-TDDFT). The results were compared with the Ab Initio Multiple Spawning strategy (also coupled to LR-TDDFT), which reproduces the branching of photoproducts observed in CT-MQC.
Basile was a co-organizer with Dr. Federica Agostini and Dr. Ari P. Seitsonen of the Extended Software Development Workshop (ESDW) in quantum dynamics, which took place in the “Maison de la Simulation” near Paris. Morning sessions were dedicated to the scientific presentations and HPC training, while coding sessions took place every afternoon. The central goal of this event was to further extend the E-CAM library of software for quantum dynamics, as well as to stimulate future developments in the field.
More information about E-CAM software repositories can be found here.
Ab Initio Multiple Spawning (AIMS) is a method that allows for accurate yet efficient excited-state molecular dynamics. It portrays nuclear wavepackets as linear combinations of coupled Gaussian functions that follow classical trajectories. (See our recent review for a detailed discussion on this method.)
AIMS emerges from the (formally-exact) Full Multiple Spawning framework by invoking two central approximations related to the coupling between Gaussian functions. While AIMS has been successfully employed to simulate the photochemistry of numerous molecules, a detailed analysis of the strengths and weaknesses of its underlying approximations was missing in the literature.
Together with Dr. Benoît Mignolet (University of Liège), we investigated in great details the approximations in AIMS using a model molecule, LiH, photoexcited by an external electric field to generate interfering nuclear wavepackets. Interestingly, AIMS could capture the behavior of properties like electronic state populations and the more challenging time-dependent molecular dipole moment, even if this low-dimensional system constitutes one of the worst-case scenarios for the AIMS approximations. We furthermore proposed and validated strategies, compatible with the simulation of larger molecular systems, to overcome some potential shortcomings of these approximations.
(On a less serious note, this article is also the first one of the group fulfilling the requirements of the Palatinate Challenge.)
This Symposium was directly followed by the first TeraChem/FMS developer meeting. TeraChem is a GPU-accelerated quantum chemical package that can be interfaced with the nonadiabatic dynamics method Full Multiple Spawning (FMS). Both codes are used in the ISP group and this developer meeting was an exciting opportunity to discuss their future directions and developments.
From February 26 to March 2, Basile co-organized a CECAM School entitled “Nonadiabatic Molecular Dynamics in Three Different Flavors” with Todd Martínez (Stanford University), Graham Worth (University College London), and Ivano Tavernelli (IBM Zürich). The School took place at the CECAM Headquarter in Lausanne (Switzerland).
The central idea of this School was to introduce the 40 participants to general concepts of computational photochemistry as well as three distinct methods for nonadiabatic molecular dynamics: Multiconfiguration Time-Dependent Hartree (MCTDH), Ab Initio Multiple Spawning (AIMS), and Trajectory Surface Hopping (TSH). The morning was dedicated to lectures for each method, and exercises on the computer were organized each afternoon. The main goal of these exercises was for the participants to play with each technique and to be able to use them, maybe, in their own research.
What happens to a molecule after it absorbs light? Have a look at our review freshly published in Chemical Reviews to discover different theoretical approaches that aim to answer this question. We focus our attention on nonadiabatic frameworks that can be derived from the time-dependent molecular Schrödinger equation and employ traveling Gaussian functions to describe nuclear wavefunctions. We discuss in details the different approximations used to produce methods that are compatible with an on-the-fly propagation of the Schrödinger equation for molecules in their full configuration space, such as Ab Initio Multiple Spawning (AIMS), MultiConfigurational Ehrenfest (MCE), or variational Multi-Configurational Gaussian (vMCG).