This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods.
View Article and Find Full Text PDFDescribing kinetic processes within a perturbation theory approach such as Fermi's golden rule requires an understanding of the initial and final states of the system. A number of different methods have been proposed for obtaining these diabatic-like states, but a robust criterion for evaluating their accuracy has not been established. Here, we approach the problem of determining the most appropriate set of diabatic states for use in incoherent rate expressions.
View Article and Find Full Text PDFWe have measured the polarizabilities of four families of molecules adsorbed to Au{111} surfaces, with structures ranging from fully saturated to fully conjugated, including single-molecule switches. Measured polarizabilities increase with increasing length and conjugation in the adsorbed molecules and are consistent with theoretical calculations. For single-molecule switches, the polarizability reflects the difference in substrate-molecule electronic coupling in the ON and OFF conductance states.
View Article and Find Full Text PDFOrganic semiconductors (OSCs) have recently received significant attention for their potential use in photovoltaic, light emitting diode, and field effect transistor devices. Part of the appeal of OSCs is the disordered, amorphous nature of these materials, which makes them more flexible and easier to process than their inorganic counterparts. In addition to their technological applications, OSCs provide an attractive laboratory for examining the chemistry of heterogeneous systems.
View Article and Find Full Text PDFWe report a simple and reproducible method for fabricating heterometallic nanogaps, which are made of two different metal nanorods separated by a nanometer-sized gap. The method is based upon on-wire lithography, which is a chemically enabled technique used to synthesize a wide variety of nanowire-based structures (e.g.
View Article and Find Full Text PDFWe present a model for the transmission of spin-polarized electrons through oriented chiral molecules, where the chiral structure is represented by a helix. The scattering potential contains a confining term and a spin-orbit contribution that is responsible for the spin-dependent scattering of electrons by the molecular target. The differential scattering cross section is calculated for right- and left-handed helices and for arbitrary electron spin polarizations.
View Article and Find Full Text PDFWe implement a method to study transport in a basis of many-body molecular states using the nonequilibrium Hubbard Green's function technique. A well-studied system, a junction consisting of benzene-dithiol on gold, is the focus of our consideration. Electronic structure calculations are carried out at the Hartree-Fock (HF), density functional theory (DFT), and coupled-cluster singles and doubles (CCSD) levels, and multiple molecular states are included in the transport calculation.
View Article and Find Full Text PDFTime-resolved transient optical absorption and EPR (TREPR) spectroscopies are used to probe the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) ((1*)PDI) with a stable tert-butylphenylnitroxide radical ((2)BPNO(*)) at specific distances and orientations. The (2)BPNO(*) radical is connected to the PDI with the nitroxide and imide nitrogen atoms either para (1) or meta (3) to one another, as well as through a second intervening p-phenylene spacer (2). Transient absorption experiments on 1-3 reveal that (1*)PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with tau approximately = 2 ps to give structurally dependent 8-31% yields of (3*)PDI.
View Article and Find Full Text PDFAngew Chem Int Ed Engl
September 2009
Click to fill the gap: The in situ modular fabrication of molecular transport junctions in nanogaps generated by on-wire lithography is achieved by using click chemistry (see picture). The formation of molecular junctions proceeds in high yields and can be used to test different molecules; the triazole group also maintains conjugation in the molecular wires. Raman spectroscopy is used to characterize the molecular assembly processes.
View Article and Find Full Text PDFWe present a simple model for analyzing the spin dynamics of a three-spin system representing a photoexcited chromophore coupled to a stable radical species. Perturbation theory yields a Fermi's Golden Rule-type rate expression that describes the formation of a local triplet on the chromophore through spin exchange with the radical. The error introduced by perturbation theory is evaluated for a number of parameters.
View Article and Find Full Text PDFThis article shows that, although Boys localization is usually applied to single-electron orbitals, the Boys method itself can be applied to many electron molecular states. For the two-state charge-transfer problem, we show analytically that Boys localization yields the same charge-localized diabatic states as those found by generalized Mulliken-Hush theory. We suggest that for future work in electron transfer, where systems have more than two charge centers, one may benefit by using a variant of Boys localization to construct diabatic potential energy surfaces and extract electronic coupling matrix elements.
View Article and Find Full Text PDFWe discuss several proposed explanations for the switching and negative differential resistance (NDR) behavior seen in some molecular junctions. Several theoretical models are discussed, and we present results of electronic structure calculations on a series of substituted oligo(phenylene ethynylene) molecules. It is shown that a previously proposed polaron model is successful in predicting NDR behavior, and the model is elaborated with image charge effects and parameters from electronic structure calculations.
View Article and Find Full Text PDFThe authors examine the connection between electron transport under bias in a junction and nonadiabatic intramolecular electron transfer (ET). It is shown that under certain assumptions it is possible to define a stationary current that allows the computation of the intramolecular transfer rate using the same formalism that is employed in the description of transport. They show that the nonequilibrium Green's function formalism of quantum transport can be used to calculate the ET rate.
View Article and Find Full Text PDFThe effect of anharmonicity in the intramolecular modes of a model system for exothermic intramolecular nonadiabatic electron transfer is probed by examining the dependence of the transition probability on the exoergicity. The Franck-Condon factor for the Morse potential is written in terms of the Gauss hypergeometric function both for a ground initial state and for the general case, and comparisons are made between the first-order perturbation theory results for transition probability for harmonic and Morse oscillators. These results are verified with quantum dynamical simulations using wave-packet propagations on a numerical grid.
View Article and Find Full Text PDF