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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Simple and General Unitarity Conserving Numerical Real-Time Propagators of the Time-Dependent Schrödinger Equation Based on Magnus Expansion

Magnus expansion (ME) provides a general way to expand the real-time propagator of a time-dependent Hamiltonian within the exponential such that the unitarity is satisfied at any order. Here, we use this property and explicit integration of Lagrange interpolation formulas for the time-dependent Hamiltonian within each time interval and derive approximations that preserve unitarity for the differential time evolution operators of general time-dependent Hamiltonians. The resulting second-order approximation is the same as using the average of Hamiltonians for two end points of time. We identify three fourth-order approximations involving commutators of Hamiltonians at different times and also derive a sixth-order expression. A test of these approximations along with other available expressions for a two-state time-dependent Hamiltonian with sinusoidal time dependences provides information on the relative performance of these approximations and suggests that the derived expressions can serve as useful numerical tools for time evolution in time-resolved spectroscopy, quantum control, quantum sensing, real-time ab initio quantum dynamics, and open system quantum dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Application of a New Ensemble Conserving Quantum Dynamics Simulation Algorithm to Liquid para-Hydrogen and ortho-Deuterium

Here, we apply the Feynman-Kleinert Quasi-Classical Wigner (FK-QCW) method developed in our previous work [Smith et al., J. Chem. Phys. 142, 244112 (2015)] for the determination of the dynamic structure factor of liquid para-hydrogen and ortho-deuterium at state points of (T = 20.0 K, n = 21.24 nm -3 ) and (T = 23.0 K, n = 24.61 nm -3 ), respectively. When applied to this challenging system, it is shown that this new FK-QCW method consistently reproduces the experimental dynamic structure factor reported by Smith et al. [J. Chem. Phys. 140, 034501 (2014)] for all momentum transfers considered. Moreover, this shows that FK-QCW provides a substantial improvement over the Feynman-Kleinert linearized path-integral method, in which purely classical dynamics are used. Furthermore, for small momentum transfers, it is shown that FK-QCW provides nearly the same results as ring-polymer molecular dynamics (RPMD), thus suggesting that FK-QCW provides a potentially more appealing algorithm than RPMD since it is not formally limited to correlation functions involving linear operators.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Sequence conservation of protein binding segments in intrinsically disordered regions

Highlights: • Sequence conservation in protein-binding segments in IDRs is analyzed. • Functionally constraint residues in protein-binding segments in IDRs are conserved. • Evolutionary distance should be considered in the comparison of IDRs. • Cryptic binding regions may influence sequence conservation of IDRs. Intrinsically disordered proteins are proteins with intrinsically disordered regions (IDRs) that do not adopt a globular structure in their free state. IDRs have unique regions having protein-binding segments of which play pivotal roles in many biological processes. The binding sites in IDRs are heterogeneous in terms of their sequence conservation: some are conserved and others are not. We have been running a database of intrinsically disordered proteins, IDEAL, and collecting such binding segments, which are called protean segments (ProSs), within it. In this study, we compared the sequence conservation of ProSs, structural domains (SDs), and IDRs other than ProSs (non-ProSs) and found that i) functionally constrained residues in ProSs tend to be conserved, ii) the distribution of conservation scores of ProSs is similar to those of SDs but not non-ProSs, and iii) ProSs found in human proteins are mostly conserved only in vertebrates. These results indicate that the conservation patterns in ProSs principally follow the general rules found in SDs. However, we need to consider evolutionary distance when comparing IDR sequences because ProSs can readily emerge and disappear over the course of protein evolution. Moreover, many ProSs may remain to be identified, which may account for the heterogeneity of the sequence conservation of IDRs.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2025

Local conservation of energy in fully implicit PIC algorithms

We consider the issue of strict, fully discrete local energy conservation for a whole class of fully implicit local-charge- and global-energy-conserving particle-in-cell (PIC) algorithms. Earlier studies demonstrated these algorithms feature strict global energy conservation. However, whether a local energy conservation theorem exists (in which the local energy update is governed by a flux balance equation at every mesh cell) for these schemes is unclear. In this study, we show that a local energy conservation theorem indeed exists. We begin our analysis with the 1D electrostatic PIC model without orbit-averaging, and then generalize our conclusions to account for orbit averaging, multiple dimensions, and electromagnetic models (Darwin). In all cases, a temporally, spatially, and particle-discrete local energy conservation theorem is shown to exist, proving that these formulations (as originally proposed in the literature), in addition to being locally charge conserving and globally energy conserving, are strictly locally energy conserving as well. In contrast to earlier proofs of local conservation in the literature, which only considered continuum time, our result is valid for the fully implicit time-discrete version of all models considered, including important features such as orbit averaging. We demonstrate the local-energy-conservation property numerically with a paradigmatic numerical example.

97 MATHEMATICS AND COMPUTING↗