20 research outputs found
Analytic nuclear forces and molecular properties from full configuration interaction quantum Monte Carlo
Unbiased stochastic sampling of the one- and two-body reduced density
matrices is achieved in full configuration interaction quantum Monte Carlo with
the introduction of a second, "replica" ensemble of walkers, whose population
evolves in imaginary time independently from the first, and which entails only
modest additional computational overheads. The matrices obtained from this
approach are shown to be representative of full configuration-interaction
quality, and hence provide a realistic opportunity to achieve high-quality
results for a range of properties whose operators do not necessarily commute
with the hamiltonian. A density-matrix formulated quasi-variational energy
estimator having been already proposed and investigated, the present work
extends the scope of the theory to take in studies of analytic nuclear forces,
molecular dipole moments and polarisabilities, with extensive comparison to
exact results where possible. These new results confirm the suitability of the
sampling technique and, where sufficiently large basis sets are available,
achieve close agreement with experimental values, expanding the scope of the
method to new areas of investigation.Comment: 11 page
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Complex Hydride Compounds with Enhanced Hydrogen Storage Capacity
The United Technologies Research Center (UTRC), in collaboration with major partners Albemarle Corporation (Albemarle) and the Savannah River National Laboratory (SRNL), conducted research to discover new hydride materials for the storage of hydrogen having on-board reversibility and a target gravimetric capacity of ≥ 7.5 weight percent (wt %). When integrated into a system with a reasonable efficiency of 60% (mass of hydride / total mass), this target material would produce a system gravimetric capacity of ≥ 4.5 wt %, consistent with the DOE 2007 target. The approach established for the project combined first principles modeling (FPM - UTRC) with multiple synthesis methods: Solid State Processing (SSP - UTRC), Solution Based Processing (SBP - Albemarle) and Molten State Processing (MSP - SRNL). In the search for novel compounds, each of these methods has advantages and disadvantages; by combining them, the potential for success was increased. During the project, UTRC refined its FPM framework which includes ground state (0 Kelvin) structural determinations, elevated temperature thermodynamic predictions and thermodynamic / phase diagram calculations. This modeling was used both to precede synthesis in a virtual search for new compounds and after initial synthesis to examine reaction details and options for modifications including co-reactant additions. The SSP synthesis method involved high energy ball milling which was simple, efficient for small batches and has proven effective for other storage material compositions. The SBP method produced very homogeneous chemical reactions, some of which cannot be performed via solid state routes, and would be the preferred approach for large scale production. The MSP technique is similar to the SSP method, but involves higher temperature and hydrogen pressure conditions to achieve greater species mobility. During the initial phases of the project, the focus was on higher order alanate complexes in the phase space between alkaline metal hydrides (AmH), Alkaline earth metal hydrides (AeH2), alane (AlH3), transition metal (Tm) hydrides (TmHz, where z=1-3) and molecular hydrogen (H2). The effort started first with variations of known alanates and subsequently extended the search to unknown compounds. In this stage, the FPM techniques were developed and validated on known alanate materials such as NaAlH4 and Na2LiAlH6. The coupled predictive methodologies were used to survey over 200 proposed phases in six quaternary spaces, formed from various combinations of Na, Li Mg and/or Ti with Al and H. A wide range of alanate compounds was examined using SSP having additions of Ti, Cr, Co, Ni and Fe. A number of compositions and reaction paths were identified having H weight fractions up to 5.6 wt %, but none meeting the 7.5 wt%H reversible goal. Similarly, MSP of alanates produced a number of interesting compounds and general conclusions regarding reaction behavior of mixtures during processing, but no alanate based candidates meeting the 7.5 wt% goal. A novel alanate, LiMg(AlH4)3, was synthesized using SBP that demonstrated a 7.0 wt% capacity with a desorption temperature of 150°C. The deuteride form was synthesized and characterized by the Institute for Energy (IFE) in Norway to determine its crystalline structure for related FPM studies. However, the reaction exhibited exothermicity and therefore was not reversible under acceptable hydrogen gas pressures for on-board recharging. After the extensive studies of alanates, the material class of emphasis was shifted to borohydrides. Through SBP, several ligand-stabilized Mg(BH4)2 complexes were synthesized. The Mg(BH4)2*2NH3 complex was found to change behavior with slightly different synthesis conditions and/or aging. One of the two mechanisms was an amine-borane (NH3BH3) like dissociation reaction which released up to 16 wt %H and more conservatively 9 wt%H when not including H2 released from the NH3. From FPM, the stability of the Mg(BH4)2*2NH3 compound was found to increase with the inclusion of NH3 groups in the inner-Mg coordination sphere, which in turn correlated with lowering the dimensionality of the Mg(BH4)2 network. Development of various Ak Tm-B-H compounds using SSP produced up to 12 wt% of H2 desorbed at temperatures of 400°C. However, the most active material can only be partially recharged to 2 wt% H2 at 220-300°C and 195 bar H2 pressure due to stable product formation. While gravimetric & volumetric targets are feasible, reversibility remains a persistent challenge
TraR, a Homolog of a RNAP Secondary Channel Interactor, Modulates Transcription
Recent structural and biochemical studies have identified a novel control mechanism of gene expression mediated through the secondary channel of RNA Polymerase (RNAP) during transcription initiation. Specifically, the small nucleotide ppGpp, along with DksA, a RNAP secondary channel interacting factor, modifies the kinetics of transcription initiation, resulting in, among other events, down-regulation of ribosomal RNA synthesis and up-regulation of several amino acid biosynthetic and transport genes during nutritional stress. Until now, this mode of regulation of RNAP was primarily associated with ppGpp. Here, we identify TraR, a DksA homolog that mimics ppGpp/DksA effects on RNAP. First, expression of TraR compensates for dksA transcriptional repression and activation activities in vivo. Second, mutagenesis of a conserved amino acid of TraR known to be critical for DksA function abolishes its activity, implying both structural and functional similarity to DksA. Third, unlike DksA, TraR does not require ppGpp for repression of the rrnB P1 promoter in vivo and in vitro or activation of amino acid biosynthesis/transport genes in vivo. Implications for DksA/ppGpp mechanism and roles of TraR in horizontal gene transfer and virulence are discussed
Mechanism of Photocatalytic Water Oxidation by Graphitic Carbon Nitride
Carbon nitride materials
are of great interest for photocatalytic
water splitting. Herein, we report results from first-principles simulations
of the specific electron- and proton-transfer processes that are involved
in the photochemical oxidation of liquid water with heptazine-based
molecular photocatalysts. The heptazine chromophore and the solvent
molecules have been described strictly at the same level of electronic
structure theory. We demonstrate the critical role of solvent molecules
for the absorption properties of the chromophore and the overall photocatalytic
cycle. A simple model is developed to describe the photochemical water
oxidation mechanism. Our results reveal that heptazine possesses energy
levels that are suitable for the water oxidation reaction. We suggest
design principles for molecular photocatalysts which can be used as
descriptors in future experimental and computational screening studies
The (<i>E</i> + <i>A</i>) × (<i>e</i> + <i>a</i>) Jahn–Teller and Pseudo-Jahn–Teller Hamiltonian Including Spin–Orbit Coupling for Trigonal Systems
The Hamiltonian describing <i>E</i> × <i>e</i> Jahn–Teller (JT) coupling
and (<i>E</i> + <i>A</i>) × (<i>e</i> + <i>a</i>) pseudo-JT
(PJT) coupling is developed beyond the standard JT theory for the
example of XY<sub>3</sub> systems, taking the bending modes of <i>a</i> and <i>e</i> symmetry into account. For the
electrostatic (spin-free) Hamiltonian, the conventional Taylor expansion
up to second order in symmetry-adapted displacements is replaced by
an expansion in invariant polynomials up to arbitrarily high orders.
The relevance of a systematic high-order expansion in the three large-amplitude
bending modes is illustrated by the construction of an eighth-order
three-sheeted three-dimensional <i>ab initio</i> potential-energy
surface for PH<sub>3</sub><sup>+</sup>. The theory of spin–orbit coupling in trigonal JT/PJT
systems is extended beyond the standard model of JT theory by an expansion
of the microscopic Breit–Pauli operator up to second order
in symmetry-adapted vibrational coordinates. It is shown that a linear <i>E</i> × <i>e</i> JT effect of relativistic origin
exists in <i>C</i><sub>3<i>v</i></sub> systems
which vanishes at the planar (<i>D</i><sub>3<i>h</i></sub>) geometry. The linear relativistic <sup>2</sup><i>E</i> – <sup>2</sup><i>A</i> PJT coupling, on the other
hand, persists at the planar geometry
Analytic nuclear forces and molecular properties from full configuration interaction quantum Monte Carlo
Induction therapy for NSCLC: A consensus report
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