seniority/Manuscript/Cover_Letter/Cover_Letter.tex

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\begin{letter}%
{To the Editors of the The Journal of Physical Chemistry Letters}

\opening{Dear Editors,}

\justifying
Please find enclosed our manuscript entitled \textit{``Hierarchy Configuration Interaction: Combining Seniority Number and Excitation Degree''}, 
which we would like you to consider as a Letter to the \textit{Journal of Physical Chemistry Letters}.
This contribution nicely fits in the \textit{``Physical Insights into Quantum Phenomena and Function''} subject category.
This contribution has never been submitted in total nor in parts to any other journal, and has been seen and approved by all authors.

Several quantum chemistry methodologies, including single- and multi-reference configuration interaction (CI) and coupled cluster, 
rely on an educated truncation of the full Hilbert space, while trying to keep most of its relevant physics.
From the birth of quantum chemistry and for many decades, most methods have been developed under an \textit{excitation}-based framework,
which are usually successful in weak correlation regimes, and, importantly, present favourable (polynomial) computational scaling.
Over the past ten years, there has been a growing interest in \textit{seniority}-based methods,
which are more suitable for strong correlation regimes, even though they display a much steeper (exponential) computational scaling.

Aiming at recovering the best of both worlds, here we present a novel partitioning of the Hilbert space, \textit{hierarchy} configuration interaction (hCI).
By benchmarking hCI against its excitation- and seniority-based parents and numerically exact results, for a series of challenging molecular systems,
and for several relevant properties, we demonstrate its overall superior convergence.
hCI inherits both the quick recovery of weak correlation and the favourable computational cost from excitation-based CI,
as well as a better account of strong correlation from seniority-based CI.

Based on the robust performance and appealing features of hCI, we anticipate our proposed strategy will both inspire further developments within the electronic structure community
and foment exploratory applications to chemically relevant systems.
Because of its novelty and impact, we expect the present study to be of interest to a wide audience within the chemistry and physics communities.
We suggest Peter Knowles, Paul Johnson, Stijn de Baerdemacker, James Shepherd, Thomas Duguet, and Gustavo Scuseria as potential referees.

We look forward to hearing from you soon.

\closing{Sincerely,}


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