abstract

Manuscript/CBD.tex

@@ -214,7 +214,7 @@
 \newcommand{\LCPQ}{Laboratoire de Chimie et Physique Quantiques (UMR 5626), Universit\'e de Toulouse, CNRS, UPS, France}
 \newcommand{\CEISAM}{Universit\'e de Nantes, CNRS,  CEISAM UMR 6230, F-44000 Nantes, France}
 
-\title{Reference energies for cyclobutadiene}
+\title{Reference Energies for Cyclobutadiene: Autoisomerization and Excited States}
 
 \author{Enzo \surname{Monino}}
 	\email{emonino@irsamc.ups-tlse.fr}
@@ -230,7 +230,12 @@
 	\affiliation{\LCPQ}
 
 \begin{abstract}
-The cyclobutadiene molecule represents a playground for ground state and excited states methods. Indeed, due to the high symmetry of the molecule, especially at the square geometry (\Dfour) but also at the rectangular structure ({\Dtwo}), the ground state and the excited states of cyclobutadiene exhibit multiconfigurational character where single reference methods such as adiabatic time-dependent density functional theory (TD-DFT) or equation-of-motion coupled cluster (EOM-CC) show difficulty to describe them. In this work we provide an extensive study of the autoisomerization barrier, where the rectangular ({\Dtwo}) and the square geometry ({\Dfour}) are needed, and of the vertical excitations energies of cyclobutadiene using a large range of methods and basis sets. In order to tackle the problem of multiconfigurational character presents in the autoisomerization barrier and in the vertical excitation energies selected configuration interaction and multireference (CASSCF, CASPT2, and NEVPT2) calculations are performed. Moreover coupled cluster calculations such as CCSD, CC3, CCSDT, CC4 and CCSDTQ are added to the set of methods. To complete the study we provide spin-flip results, which are known to give correct description of multiconfigurational character states, in the TD-DFT framework where numerous exchange-correlation functionals are considered, we also add algebraic diagrammatic construction (ADC) calculations in the spin-flip formalism where we use the ADC(2)-s, ADC(2)-x and ADC(3) schemes. A theoretical best estimate is defined for the autoisomerization barrier and for each vertical energies.
+The cyclobutadiene molecule is a well-known playground for theoretical chemists and is particularly suitable to test ground- and excited-state methods.
+Indeed, due to its high spatial symmetry, especially at the $D_{4h}$ square geometry but also at the $D_{2h}$ rectangular structure, the ground and excited states of cyclobutadiene exhibit multiconfigurational characters and single-reference methods, such as adiabatic time-dependent density functional theory (TD-DFT) or equation-of-motion coupled cluster (EOM-CC), are notoriously known to struggle in such situations. 
+In this work, using a large panel of methods and basis sets, we provide an extensive computational study of the autoisomerization barrier (defined as the difference between the square and rectangular ground-state energies) and the vertical excitation energies of the $D_{2h}$ and $D_{4h}$ structures. 
+In particular, selected configuration interaction (SCI), multireference perturbation theory (CASSCF, CASPT2, and NEVPT2), and coupled cluster (CCSD, CC3, CCSDT, CC4, and CCSDTQ) calculations are performed. 
+The spin-flip formalism, which is known to provide a correct description of states with multiconfigurational character, is also tested within TD-DFT (where numerous exchange-correlation functionals are considered) and the algebraic diagrammatic construction [ADC(2)-s, ADC(2)-x, and ADC(3)]. 
+A theoretical best estimate is defined for the autoisomerization barrier and for each vertical transition energy.
 \end{abstract}
 
 \maketitle