The generalized energy-based fragmentation (GEBF) approach (Li, W.; Li, S.; Jiang, Y. J. Phys. Chem. A 2007, 111, 2193) is extended for geometry optimizations and vibrational spectra calculations of general large molecules or clusters. In this approach, the total energy and its derivatives, and some molecular properties, of a target system are obtained from conventional calculations on a series of subsystems derived from the target system. Each subsystem is electronically embedded in the background point charges generated by all other atoms outside the subsystem so that the long-range interactions and polarization effects between remote fragments are approximately taken into account. The approach computationally scales linearly with the system size and can be easily implemented for large-scale parallelization. By comparing the results from the conventional and GEBF calculations for several test molecules including a polypeptide and a water cluster, we demonstrate that the GEBF approach is able to provide quite reliable predictions for molecular geometries, vibrational frequencies, and thermochemistry data and satisfactory descriptions for vibrational intensities, for general molecules with polar or charged groups.

An efficient implementation of the generalized energy-based fragmentation (GEBF) approach (Li, W.; Li, S.; Jiang, Y. J Phys. Chem. A 2007, 111, 2193) for treating a wide range of large molecules is presented. In this implementation, the fragmentation process can be automatically done for a general molecule, with only some functional groups defined by users. A new and fast scheme is designed for the generation of various subsystems and the derivation of their coefficients. The newly implemented GEBF approach has been applied to several large molecules including proteins, nucleic acids, and supermolecules with fused aromatic rings. Test calculations within the Hartree-Fock (HF) and density functional theory (DFT) framework demonstrate that the GEBF approach can provide reasonably accurate ground-state energies and optimized structures, which are in good agreement with those from conventional HF or DFT calculations. The GEBF approach implemented in this work can now be employed by nonexpert users to compute energies, optimized structures, and some molecular properties at various ab initio levels for a broad range of large molecules on ordinary PC workstations.

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Accurate N1s and C1s XPS spectra of g-C3N4 were obtained by a combined cluster-periodic approach and we make new assignments.

A large protonated water cluster, H+(H2O)27, has been trapped and stabilized within the well-modulated cavity of a 3D metal-organic framework formed by cobalt(II) and 4,4'-bipyridine-N,N'-dioxide with a globular Keggin structure [PW12O4]3- anion as template. The structurally characterized protonated water cluster might comprise a (H2O)26 shell with Oh symmetry and a monowater core within the center which is suggested to be a hydronium ion with the Eigen model.

Measuring the nonlinear response of electrons and nuclei to attosecond broadband X-ray radiation has become possible by newly developed free electron lasers and high harmonic generation light sources. The design and interpretation of these novel experiments poses considerable computational challenges. In this chapter we survey the basic description of nonlinear X-ray spectroscopy signals and the electronic structure protocols which may be used for their simulation.

Harvesting non-emissive spin-triplet charge-transfer (CT) excitons of organic semiconductors is fundamentally important for increasing the operation efficiency of future devices.H ere we observe thermally activated delayed fluorescence (TADF) in a1 :2 CT cocrystal of trans-1,2-diphenylethylene (TSB) and 1,2,4,5-tetracyanobenzene (TCNB). This cocrystal system is characterized by absorption spectroscopy, variable-temperature steady-state and time-resolved photoluminescence spectroscopy, single-crystal X-ray diffraction, and first-principles calculations.T hese data reveal that intermolecular CT in cocrystal narrows the singlet-triplet energy gap and therefore facilitates reverse intersystem crossing (RISC) for TADF.T hese findings open up an ew way for the future design and development of novel TADF materials.Improving the exciton-utilizing efficiency is an important issue in organic electronics.L ots of research attention has been focused on designing new efficient organic light-emitting diode (OLED), organic light-emitting transistor (OLET), and organic photovoltaic (OPV) cell materials. During the state-of-the-art device operations,t he generated singlet and triplet excitons from charge recombination have ar atio of 1:3a ccording to the spin statistics. [1,2] For fluorescence-based devices,o nly singlet excitons are spinallowed, and light can be emitted with am aximum excitonutilizing efficiency of 25 %. In contrast, phosphorescencebased devices can utilize nearly 100 %excitons through the so called "heavy atom effect" -e nhanced intersystem crossing (ISC) from the singlet to the triplet state.H owever,s uch devices commonly use noble-metals,such as Ir III and Pt II ,and their high cost, scarcity,a nd toxicity hinder their long-term development and industrial applications. [3,4] To date,m uch effort has been devoted to breaking through the exciton statistics limit by using rare-earth-metal-free materials. [5,6] To improve the exciton-utilizing efficiency,h arvesting nonemissive triplet excitons of organic semiconductors is of fundamental interest. However,p ure organic compounds hardly exhibit room temperature phosphorescence (RTP) for two reasons:G round-state excimers may quench at high concentration or in the solid state due to aggregation effects; and triplet-state excimers are easy to quench due to their sensitivity to ambient conditions,i ncluding oxygen gas and humidity. [7][8][9] Alternatively,s cientists have been working on harvesting excitons based on reversed ISC (RISC) between different excited state manifolds.A dachisg roup proposed the thermally activated delayed fluorescence (TADF) mechanism through organic molecular design, wherein the system evolves from at riplet excited state to the lowest singlet excited state (S 1 )via RISC. [10,11] Masgroup discovered ahot exciton RISC mechanism with hybridized local and charge transfer (HLCT) character. [12,13] In both cases,the CT nature of excited states induces spatial separation in orbitals and as mall singlet-triplet energy gap (DE ST ,u sually appro...

A simple atom-replacement approach is proposed for estimating the individual contributions of each intermolecular hydrogen bond (HB) in multiple hydrogen-bonded systems. The approach is validated by calculations on the homodimer of formylformamide and then applied to nucleic acid base pairs (adenine-thymine and guanine-cytosine) and some quadruply hydrogen-bonded dimers. With the help of this method, it is easy to distinguish the relative strength of each HB, and identify the main factors contributing to the total binding energies of multiple HBs.