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Register a new userPhotoinduced dynamics of organic molecules using nonequilibrium Greens functions with second-Born, $GW$, $T$-matrix and three-particle ladder correlations
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The ultrafast hole dynamics triggered by the photoexcitation of molecular targets is a highly correlated process even for those systems, like organic molecules, having a weakly correlated ground state. We here provide a unifying framework and a numerically efficient matrix formulation of state-of-the-art non-equilibrium Greens function (NEGF) methods like second-Born as well as $GW$ and $T$-matrix without and {em with} exchange diagrams. Numerical simulations are presented for a paradigmatic, exactly solvable molecular system and the shortcomings of the established NEGF methods are highlighted. We then develop a NEGF scheme based on the Faddeev treatment of three-particle correlations; the exceptional improvement over established methods is explained and demonstrated. The Faddeev NEGF scheme scales linearly with the maximum propagation time, thereby opening prospects for femtosecond simulations of large molecules.
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One-particle Greens function methods can model molecular and solid spectra at zero or non-zero temperatures. One-particle Greens functions directly provide electronic energies and one-particle properties, such as dipole moment. However, the evaluation of two-particle properties, such as $langle{S^2}rangle$ and $langle{N^2}rangle$ can be challenging, because they require a solution of the computationally expensive Bethe--Salpeter equation to find two-particle Greens functions. We demonstrate that the solution of the Bethe--Salpeter equation can be complitely avoided. Applying the thermodynamic Hellmann--Feynman theorem to self-consistent one-particle Greens function methods, we derive expressions for two-particle density matrices in a general case and provide explicit expressions for GF2 and GW methods. Such density matrices can be decomposed into an antisymmetrized product of correlated one-electron density matrices and the two-particle electronic cumulant of the density matrix. Cumulant expressions reveal a deviation from ensemble representability for GW, explaining its known deficiencies. We analyze the temperature dependence of $langle{S^2}rangle$ and $langle{N^2}rangle$ for a set of small closed-shell systems. Interestingly, both GF2 and GW show a non-zero spin contamination and a non-zero fluctuation of the number of particles for closed-shell systems at the zero-temperature limit.
We combine the renormalized singles (RS) Greens function with the T-Matrix approximation for the single-particle Greens function to compute quasiparticle energies for valence and core states of molecular systems. The $G_{text{RS}}T_0$ method uses the RS Greens function that incorporates singles contributions as the initial Greens function. The $G_{text{RS}}T_{text{RS}}$ method further calculates the generalized effective interaction with the RS Greens function by using RS eigenvalues in the T-Matrix calculation through the particle-particle random phase approximation. The $G_{text{RS}}T_{text{RS}}$ method provides significant improvements over the one-shot T-Matrix method $G_0T_0$ as demonstrated in calculations for GW100 and CORE65 test sets. It also systematically eliminates the dependence of $G_{0}T_{0}$ on the choice of density functional approximations (DFAs). For valence states, the $G_{text{RS}}T_{text{RS}}$ method provides an excellent accuracy, which is better than $G_0T_0$ with Hartree-Fock (HF) or other DFAs. For core states, the $G_{text{RS}}T_{text{RS}}$ method correctly identifies desired peaks in the spectral function and significantly outperforms $G_0T_0$ on core level binding energies (CLBEs) and relative CLBEs, with any commonly used DFAs.
Due to non-linear structure, iterative Greens function methods can result in multiple different solutions even for simple molecular systems. In contrast to the wave-function methods, a detailed and careful analysis of such molecular solutions was not performed before. In this work, we use two-particle density matrices to investigate local spin and charge correlators that quantify the charge-resonance and covalent characters of these solutions. When applied within unrestricted orbital set, spin correlators elucidate the broken symmetry of the solutions, containing necessary information for building effective magnetic Hamiltonians. Based on GW and GF2 calculations of simple molecules and transition metal complexes, we construct Heisenberg Hamiltonians, four-spin-four-center corrections, as well as biquadratic spin-spin interactions. These Hamiltonian parametrizations are compared to prior wave-function calculations.
Nano-crystalline diamond is a new carbon phase with numerous intriguing physical and chemical properties and applications. Small doped nanodiamonds for example do find increased use as novel quantum markers in biomedical applications. However, growing doped nanodiamonds below sizes of 5 nm with controlled composition has been elusive so far. Here we grow nanodiamonds under conditions where diamond-like organic seed molecules do not decompose. This is a key first step toward engineered growth of fluorescent nanodiamonds wherein a custom designed seed molecule can be incorporated at the center of a nanodiamond. By substituting atoms at particular locations in the seed molecule it will be possible to achieve complex multi-atom diamond color centers or even to engineer complete nitrogen-vacancy (NV) quantum registers. Other benefits include the potential to grow ultrasmall nanodiamonds, wherein each diamond no matter how small can have at least one bright and photostable fluorescent emitter.
Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technological applications, are widely studied and therefore provide an excellent system for benchmarking theoretical methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approximation relative to new high-quality CCSD(T) reference data for charged excitations of the acenes. We compare GW calculations with a number of hybrid density functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with molecule length increasing from benzene to hexacene. We find that GW calculations with starting points based on an optimally tuned range-separated hybrid (OTRSH) density functional and eigenvalue self-consistency can yield quantitative ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from reference values. Our work paves the way for predictive and cost-effective GW calculations of charged excitations of molecules and identifies certain limitations of current GW methods used in practice for larger molecules.
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