State of the Art of Technology for Rural Water System Development

The objective of this study was to review the current state of the art in rural water system technology. This was to be accomplished by a literature review of the Water Resources Scientific Information Center (WRSIC), National Technical Information Service (NTIS), and Smithsonian Science Information Exchange (SSIE). This literature search was to be augmented by interviews with consulting engineers, operating system managers and industrial literature. Both groundwater and surface water technology was to be reviewed. The technology was then to be grouped into four classes: (1) current technology, (2) technology which has been developed, but not currently being used, (3) technology under development which looks promising, and (4) research needed. After the searches were conducted, it became feasible to combine group (2) and (3) into one group entitled technology under development but not fully utilized

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

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. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

Exploring Non-Condon Effects in a Covalent Tetracene Dimer: How Important Are Vibrations in Determining the Electronic Coupling for Singlet Fission?

Singlet fission (SF) offers opportunities for wavelength-selective processing of solar photons with an end goal of achieving higher efficiency inexpensive photovoltaic or solar-fuels-producing devices. In order to evaluate new molecular design strategies and for theoretical exploration of dynamics, it is important to put in place tools for efficient calculation of the electronic coupling between single-exciton reactant and multiexciton product states. For maximum utility, the couplings should be calculated at multiple nuclear geometries (rather than assumed constant everywhere, i.e., the Condon approximation) and we must be able to evaluate couplings for covalently linked multichromophore systems. With these requirements in mind, here we discuss the simplest methodology possible for rapid calculation of diabatic one-electron coupling matrix elementsbased on Boys localization and rediagonalization of molecular orbitals. We focus on a covalent species called BT1 that juxtaposes two tetracene units in a partially cofacial geometry via a norbornyl bridge. In BT1, at the equilibrium <i>C</i>_2v structure, the “nonhorizontal” couplings between HOMOs and LUMOs (<i>t</i>_HL and <i>t</i>_LH) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of <i>t</i>_AB coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the <i>C</i>_2v point group) that turn on <i>t</i>_HL and <i>t</i>_LH values as well as for the patterns that emerge in constructive versus destructive interference of pathways to the SF product. For the best modes, calculated electronic coupling magnitudes for SF (at root-mean-squared deviation in position at 298 K), are within a factor of 2 of that seen for noncovalent tetracene dimers relevant to the molecular crystal. An overall “effective” electronic coupling is also given, based on the Stuchebrukhov formalism for non-Condon electron transfer rates

Ultrafast Electronic Relaxation through a Conical Intersection: Nonadiabatic Dynamics Disentangled through an Oscillator Strength-Based Diabatization Framework

We employ surface hopping trajectories to model the short-time dynamics of gas-phase and partially solvated 4-(<i>N</i>,<i>N</i>-dimethylamino)­benzonitrile (DMABN), a dual fluorescent molecule that is known to undergo a nonadiabatic transition through a conical intersection. To compare theory vs time-resolved fluorescence measurements, we calculate the mixed quantum–classical density matrix and the ensemble averaged transition dipole moment. We introduce a diabatization scheme based on the oscillator strength to convert the TDDFT adiabatic states into diabatic states of L_a and L_b character. Somewhat surprisingly, we find that the rate of relaxation reported by emission to the ground state is almost 50% slower than the adiabatic population relaxation. Although our calculated adiabatic rates are largely consistent with previous theoretical calculations and no obvious effects of decoherence are seen, the diabatization procedure introduced here enables an explicit picture of dynamics in the branching plane, raising tantalizing questions about geometric phase effects in systems with dozens of atoms