Computational models can improve our understanding of photochemistry, where light interacts with molecules to drive chemical reactions. Multireference theories based on configuration interaction (CI) accurately describe the electronic structure of photochemical reactions. This work describes two improvements to full CI that enable simulation of complex photochemical processes. Photochemical reactions with a single reaction coordinate can be accurately simulated with small orbital active spaces. More complicated reactions may require correlation of additional electrons. The seminal graphics processing unit (GPU) implementation of a CI algorithm enabled routine use of wave functions with more than 150 million basis functions - bettering alternative approaches by more than 10x. In the present work, I describe advances that extend GPU CI by yet another order of magnitude, allowing for time-dependent simulation of molecules having multiple reaction coordinates. Time-dependent simulation explores nonequilibrium molecular geometries. Degeneracies between electronic states are often encountered in such simulations. These degeneracies can hinder convergence in Slater determinant (SD) basis CI calculations, and nearly four decades of effort has failed to provide a solution to the problem. Despite these challenges, the SD basis is often preferred because it is more computationally efficient than the more robust configuration state function (CSF) basis. To leverage advantages of both the SD the CSF bases, we have developed a GPU CSF/SD basis transformation that enables a hybrid-basis CI program, simultaneously solving the convergence problem and reducing the size of the CI basis set by more than 2x. Taken together, GPU CI algorithms and hybrid CSF/SD basis CI diagonalization enable the efficient simulation of complex molecular photochemistry where as many as 18 electrons in 18 orbitals are correlated.

First principles configuration interaction simulation approaches for exploring photochemistry

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.