Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Abstract

Spectroscopic and optical properties of nanosystems and point defects are discussed within the framework of Green's function methods. We use an approach based on evaluating the self-energy in the so-called GW approximation and solving the Bethe-Salpeter equation in the space of singleparticle transitions. Plasmon-pole models or numerical energy integration, which have been used in most of the previous GW calculations, are not used. Fourier transforms of the dielectric function are also avoided. This approach is applied to benzene, naphthalene, passivated silicon clusters (containing more than one hundred atoms), and the F center in LiCl. In the latter, excitonic effects and the 1s → 2p defect line are identified in the energy-resolved dielectric function. We also compare optical spectra obtained by solving the Bethe-Salpeter equation and by using time-dependent density functional theory in the local, adiabatic approximation. From this comparison, we conclude that both methods give similar predictions for optical excitations in benzene and naphthalene, but they differ in the spectra of small silicon clusters. As cluster size increases, both methods predict very low cross section for photoabsorption in the optical and near ultra-violet ranges. For the larger clusters, the computed cross section shows a slow increase as function of photon frequency. Ionization potentials and electron affinities of molecules and clusters are also calculated.

Structural and excited-state properties of oligoacene crystals from first principles

“…Molecules are modeled in a large supercell with dimensions chosen to contain 99% of the HOMO (see Supplemental Material [100] for details), with the internal coordinates relaxed using PBE. We use the static-remainder technique to accelerate the convergence with number of bands [101], using the version of Deslippe et al [102]. A Wigner-Seitz Coulomb truncation scheme is used to eliminate interactions between molecules of neighboring cells in the periodic lattice [91].…”

molgw 1: Many-body perturbation theory software for atoms, molecules, and clusters

“…Its first-order approximation, namely the GW approximation, has been used with great success for the last 30 years in extended systems to calculate the band gaps of solids [6,7,8,9]. In the past, the application of the GW approximation to finite systems was rather infrequent [10,11,12,13]. But there has been much recent increase in interest [14,15,16,17,18,19,20,21,22,23] in the application of the GW approximation to problems involving atoms, molecules, and clusters, in part driven by the quest to develop efficient techniques to address mixed systems such as molecular junctions [24,25,26,27,28].…”

“…Implementations of the GW approximation of MBPT for finite systems are much rarer, but their numbers have increased during the last few years and include rgwbs [13], fhi-aims [36], fiesta [15], turbomol [37], and molgw. While molgw has been employed in several previous studies [18,23,38,39,40], it has never been fully described in a single text.…”

“…In the recent past, several techniques have been designed to account for the states that are neglected in the context of plane-wave approaches [13,98,99]. We propose to introduce the single pole approximation (SPA) [73] for the high energy states to reduce the C matrix dimension.…”

Generalization of Dielectric-Dependent Hybrid Functionals to Finite Systems

“…Functionals adopted by the condensed matter physics community in the early days of DFT applications to solids were strictly dependent on the electronic density [19,20]; those employed by the chemistry community were instead often defined using linear combinations of Hartree-Fock exchange and density-dependent exchange functionals, which are referred to as hybrid functionals. The latter have also seen many implementations and uses [11,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] in condensed matter physics. One successful example of hybrid functionals is that of the optimally tuned rangeseparated hybrid [39], defined using parameters selfconsistently tuned to enforce the validity of Koopman's theorem [40] for molecules.…”