According to recent ab initio calculations, the energy gap between the two oppositely polarized charge-transfer (CT) states of a model pentacene dimer is anomalously large, attaining 0.8 eV. Here we introduce the self-consistent charge field approach to evaluate the electrostatic stabilization energies of the pertinent states in the pentacene crystal represented by a dedicated multiscale model system containing the model dimer as its core. We demonstrate that, contrary to common wisdom, the lower of the two CT states is barely affected by the crystalline environment whereas the upper one undergoes a large red shift. Effectively, embedding of the dimer in the crystal bulk reduces the pertinent splitting by an order of magnitude, because most of the intradimer charge−quadrupole interactions are compensated by similar interactions with surrounding molecules. This resolves the apparent contradiction between the ab initio result obtained for the dimer and the splitting of about 0.04 eV resulting from microelectrostatic calculations for the crystal. ■ INTRODUCTIONThe discovery of spontaneous singlet exciton fission (SF) in solid pentacene prompted widespread interest in the electronic properties of this system, in the recent years triggering a surge of theoretical papers that strive to unravel the underlying physical mechanisms. 1−4 Various methodologies have been invoked for this purpose.No matter how vibronic coupling is treated and how the dynamic aspects of the problem are described, 1−3 the process is ultimately controlled by the energetics of the electronic states involved. Yet, as the small energy gaps between the relevant eigenstates are inevitably sensitive to the details of the wave function, the requisite accuracy is just on the verge of the predictive power of the most sophisticated quantum chemistry tools available to date. By these standards, the pentacene molecule is large. Moreover, in order to describe the fission process, it is necessary to take into account at least the two molecules on which the two emerging triplets are located, which makes a dimer the minimum model system to be considered in this context. At the ab initio level, this creates a numerical problem of formidable complexity, practically ruling out inclusion of any elements of the crystal environment in which the pertinent pair of molecules is in reality embedded.In a number of papers 1−3 focusing on diverse aspects of the fission phenomenon, various versions of the dimer model were adopted. In most cases, parametrization of the model Hamiltonian was in principle based on quantum chemistry calculations but fine-tuned on intuitive grounds to account for simplifications of the model and for potential intrinsic errors of the applied quantum chemistry methods.The paper by Zeng et al. 4 stands out from this collection, representing a consistent ab initio approach at a highly advanced level. However, in view of the accuracy it offers, even minor energy shifts resulting from the simplifications of the underlying model may significantly influen...

Photovoltaic yield is normally limited to at most two charge carriers per photon. In solid pentacene this limit may be potentially bypassed owing to singlet exciton fission into a pair of triplets. The process occurs via a superexchange mechanism mediated by charge-transfer (CT) configurations and is sensitive to their energies. As demonstrated recently, these strongly depend on the local environment of the two molecules on which the charges reside. Using a multiscale model, here we show that in the crystal bulk approximate local symmetry affects CT state energetics in a way unfavorable for fission, so that at the places where this symmetry is broken the fission probability is enhanced by up to an order of magnitude. These fission-favorable locations entail the vicinity of vacancies, specific impurities, and interfaces, such as crystallite boundaries. Hence, photovoltaic yield might be substantially increased by using nanoscopically disordered pentacene rather than highly ordered specimens.

We investigate the evolution of spin and orbital order in undoped LaMnO3 under increasing temperature with a model including both superexchange and Jahn-Teller interactions. We used several cluster mean field calculation schemes and find coexisting A-type antiferromagnetic (A-AF) and C-type alternating orbital order at low temperature. The value of the Jahn-Teller coupling between strongly correlated eg orbitals is estimated from the orbital transition temperature at TOO 780 K. By a careful analysis of on-site and on-bond correlations we demonstrate that spinorbital entanglement is rather weak. We have verified that the magnetic transition temperature is influenced by entangled spin-orbital operators as well as by entangled orbital operators on the bonds but the errors introduced by decoupling such operators partly compensate each other. Altogether, these results justify why the commonly used disentangled spin-orbital model is so successful in describing the magnetic properties and the temperature dependence of the optical spectral weights for LaMnO3. Published in: Physical Review B 94, 214426 (2016).

Singlet exciton fission in pentacene is commonly described in terms of the dimer model, containing some adjustable parameters that implicitly account for the influence of the crystalline environment. Here we use state-of-the-art results published by other authors to base the dimer model on strictly ab initio input, and consistently extend it, explicitly including the interaction with the surrounding crystal matrix. In the multiscale approach that ensues, the molecular pair residing at the center of the model cluster is identified with the dimer for which the ab initio results are available. The main environmental effects are attributed to electrostatic stabilization of the dimer charge transfer (CT) states and to CT coupling between the dimer and the adjacent molecules. According to our calculations, this latter coupling stabilizes the lowest Frenkel state so that its energy becomes lower than that of the triplet-pair (tt) state, in dramatic contrast to the situation in an isolated dimer. The interpretationally successful level order assumed in semiempirical approaches may be restored by extending the purely electronic approach of the original ab initio calculations to include vibronic effects. The results show that extreme caution must be exercised when extrapolating dimer results to draw conclusions concerning the situation in the crystal.

The model developed for LaMnO3 addresses the spin-orbital order by superexchange and Jahn-Teller orbital interactions in the cubic (perovskite) symmetry up to now whereas real crystal structure is strongly deformed. We identify and explain three a priori important physical effects arising from tetragonal deformation: (i) the splitting of eg orbitals ∝ Ez, (ii) the directional renormalization of d − p hybridization t pd , and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO3 crystal we evaluate their magnitude. It is found that the major effects of deformation are enhanced amplitude of x 2 − y 2 orbitals induced in the orbital order by Ez 300 meV and anisotropic t pd 2.0 (2.35) eV along the ab (c) cubic axis, in very good agreement with the Harrison's law. We show that the tetragonal model analyzed within mean field approximation provides a surprisingly consistent picture of the ground state. Excellent agreement with the experimental data is obtained simultaneously for: (i) eg orbital mixing angle, (ii) spin exchange constants, and (iii) the temperatures of spin and orbital phase transition. arXiv:1801.01419v1 [cond-mat.str-el]