On the nature of the oligoacene ground state

The nature of the oligoacene ground state - its spin, singlet-triplet gap, and diradical character as a function of chain-length - is a question of ongoing theoretical and experimental interest with notable technological implications. Previous computational studies have given inconclusive answers to this challenging electronic structure problem (see e.g. [1]). In the present study we exploit the capabilities of the local \textit{ab initio} Density Matrix Renormalization Group (DMRG) [2], which allows the numerically exact (FCI) solution of the Schr\"{o}dinger equation in a chosen 1-particle basis and active space for quasi-one-dimensional systems. We compute the singlet-triplet gap from first principles as a function of system length ranging from naphthalene to tetradecacene, correlating the full $\pi $-space (i.e. up to 58 electrons in 58 orbitals) and converging the results to a few $\mu $E$_{h}$ accuracy [3]. In order to study the diradical nature of the oligoacene ground state we calculate expectation values over different diradical occupation and pair-correlation operators. Furthermore we study the natural orbitals and their occupation. [1] Bendikov, Duong, Starkey, Houk, Carter, Wudl, \textit{JACS} 126 (\textbf{2004}), 7416. [2] Hachmann, Cardoen, Chan, \textit{JCP }125 (\textbf{2006}), 144101. [3] Hachmann, Dorando, Avil\'{e}s, Chan, \textit{in preparation}.