Prediction challenge: First principles simulation of the ultrafast electron diffraction spectrum of cyclobutanone.

Computer simulation has long been an essential partner of ultrafast experiments, allowing the assignment of microscopic mechanistic detail to low-dimensional spectroscopic data. However, the ability of theory to make a priori predictions of ultrafast experimental results is relatively untested. Herein, as a part of a community challenge, we attempt to predict the signal of an upcoming ultrafast photochemical experiment using state-of-the-art theory in the context of preexisting experimental data. Specifically, we employ ab initio Ehrenfest with collapse to a block mixed quantum-classical simulations to describe the real-time evolution of the electrons and nuclei of cyclobutanone following excitation to the 3s Rydberg state. The gas-phase ultrafast electron diffraction (GUED) signal is simulated for direct comparison to an upcoming experiment at the Stanford Linear Accelerator Laboratory. Following initial ring-opening, dissociation via two distinct channels is observed: the C3 dissociation channel, producing cyclopropane and CO, and the C2 channel, producing CH2CO and C2H4. Direct calculations of the GUED signal indicate how the ring-opened intermediate, the C2 products, and the C3 products can be discriminated in the GUED signal. We also report an a priori analysis of anticipated errors in our predictions: without knowledge of the experimental result, which features of the spectrum do we feel confident we have predicted correctly, and which might we have wrong?

Floquet Time-Dependent Configuration Interaction for Modeling Ultrafast Electron Dynamics.

Time-dependent electronic structure methods are a valuable tool for simulating spectroscopic experiments. Recent advances in time-dependent configuration interaction (TDCI) algorithms have made them an attractive means of modeling many-electron dynamics, particularly for cases where multireference effects are essential. Here we present an extension to TDCI, Floquet TDCI, where the electronic wave function is expanded in a basis of light-dressed determinants. Our approach is based on our high-performance graphics processing unit (GPU) accelerated implementation of complete active space configuration interaction (CASCI). Simulations of two-photon absorption demonstrate that Floquet TDCI is well-suited for modeling dynamics in intense, ultrashort laser pulses. Accurate results are obtained for pulse energies up to ∼4 × 10-4 J/cm2 per pulse in the most difficult case explored here. By simulation of a set of molecules under continuous wave coupling, we demonstrate the ability of Floquet to describe the entanglement of light and multiple molecules in a cavity (i.e., a cavity polariton). Excellent computational performance is observed: a 320 fs propagation of a large dye (C30N2H22) with a 2 as timestep and a large active space (10 electrons in 11 orbitals), including a monochromatic pulse with three photon states, was performed in 3 h 6 min on a single Tesla V100 GPU. Our Floquet TDCI algorithm scales linearly with the number of photon states and exponentially with the number of photon colors included in the calculation. We argue that its energy-conserving nature makes Floquet TDCI well-suited to drive nonadiabatic molecular dynamics simulations.

CAS without SCF-Why to use CASCI and where to get the orbitals.

The complete active space self-consistent field (CASSCF) method has seen broad adoption due to its ability to describe the electronic structure of both the ground and excited states of molecules over a broader swath of the potential energy surface than is possible with the simpler Hartree-Fock approximation. However, it also has a reputation for being unwieldy, computationally costly, and un-black-box. Here, we discuss a class of alternatives, complete active space configuration interaction (CASCI) methods, paying particular attention to their application to electronic excited states. The goal of this Perspective is fourfold. First, we argue that CASCI is not merely an approximation to CASSCF, in that it can be designed to have important qualitative advantages over CASSCF. Second, we present several insights drawn from our experience experimenting with different schemes for computing orbitals to be employed in CASCI. Third, we argue that CASCI is well suited for application to nanomaterials. Finally, we reason that, with the rise in new low-scaling approaches for describing multireference systems, there is a greater need than ever to develop new methods for defining orbitals that provide an efficient and accurate description of both static correlation and electronic excitations in a limited active space.