{To the Editors of the Journal of Chemical Theory and Computation}
\opening{Dear Editors,}
\justifying
Please find enclosed our manuscript entitled \textit{``Spin-Conserved and Spin-Flip Optical Excitations From the Bethe-Salpeter Equation Formalism''},
which we would like you to consider as a Regular Article in the \textit{Journal of Chemical Theory and Computation}.
This contribution nicely fits in the \textit{``Quantum Electronic Structure''} section.
This contribution has never been submitted in total nor in parts to any other journal, and has been seen and approved by all authors.
In the present manuscript, we extent the Bethe-Salpeter equation (BSE) approach from many-body perturbation theory to the spin-flip formalism in order to access double excitations in molecular systems. Going beyond the ubiquitous static approximation (which is equivalent to the adiabatic approximation of TD-DFT), we also propose a perturbative treatement to take into account the dynamical nature of the Bethe-Salpeter equations.
All of these techniques are combined within the spin-unrestricted formalism that we have extended to the $GW$ approximation and the BSE formalism in the present study.
Explicit equations for all of these methods are explicitly provided.
Taking the beryllium atom, the dissociation of the hydrogen molecule, and cyclobutadiene in two different geometries as examples, we show that the spin-flip BSE formalism can accurately model double excitations and seems to surpass systematically its spin-flip TD-DFT parent.
Because of its novelty and impact in the electronic structure community, we expect the present study to be of interest to a wide audience within the chemistry and physics communities.
We suggest Anna Krylov, David Casanova, So Hirata, Nicholas Mayhall, Miquel Huix-Rotllant, Patrick Rinke, Fabien Bruneval, Wim Klopper, and Lucia Reining as potential referees.
