Eunji Sim

Ultrathin Zirconium Disulfide Nanodiscs

We present a colloidal route for the synthesis of ultrathin ZrS(2) (UT-ZrS(2)) nanodiscs that are ~1.6 nm thick and consist of approximately two unit cells of S-Zr-S. The lateral size of the discs can be tuned to 20, 35, or 60 nm while their thickness is kept constant. Under the appropriate conditions, these individual discs can self-assemble into face-to-face-stacked structures containing multiple discs. Because the S-Zr-S layers within individual discs are held together by weak van der Waals interactions, each UT-ZrS(2) disc provides spaces that can serve as host sites for intercalation. When we tested UT-ZrS(2) discs as anodic materials for Li(+) intercalation, they showed excellent nanoscale size effects, enhancing the discharge capacity by 230% and greatly improving the stability in comparison with bulk ZrS(2). The nanoscale size effect was especially prominent for their performance in fast charging/discharging cycles, where an 88% average recovery of reversible capacity was observed for UT-ZrS(2) discs with a lateral diameter of 20 nm. The nanoscale thickness and lateral size of UT-ZrS(2) discs are critical for fast and reliable intercalation cycling because those dimensions both increase the surface area and provide open edges that enhance the diffusion kinetics for guest molecules.

Understanding and Reducing Errors in Density Functional Calculations

We decompose the energy error of any variational density functional theory calculation into a contribution due to the approximate functional and that due to the approximate density. Typically, the functional error dominates, but in many interesting situations the density-driven error dominates. Examples range from calculations of electron affinities to preferred geometries of ions and radicals in solution. In these abnormal cases, the error in density functional theory can be greatly reduced by using a more accurate density. A small orbital gap often indicates a substantial density-driven error.

Filtered propagator functional for iterative dynamics of quantum dissipative systems

Abstract We describe a Fortran program which calculates the reduced density matrix of a one-dimensional quantum mechanical continuous or discrete system coupled to a harmonic dissipative environment. The algorithm is based on Feynmans path integral formulation of time-dependent quantum mechanics. An adiabatic reference is employed to obtain accurate propagators and the harmonic bath is replaced by an influence functional which is discretized by optimal discrete variable representations. A propagator functional of statistically significant path segments is constructed which allows iterative evaluation of the path integral over long time periods. High efficiency is achieved with the aid of sorting and filtering criteria. The appended program is executable in either serial or parallel mode.

Reversible transformation of helical coils and straight rods in cylindrical assembly of elliptical macrocycles.

We have demonstrated that, as the molecular length of elliptical macrocycle is increased, the self-assembled structure changes from spherical micelles to helical coils and finally to monolayered vesicles, in the order of decreasing interfacial curvature. Notably, the helical coils reversibly transform into straight rods upon heating while maintaining the supramolecular chirality. This structural transition is accompanied by conformational change of the elliptical macrocycles from a boat conformation to a more planar conformation.

Biased helical folding of chiral oligoindole foldamers.

Oligoindole-based chiral foldamers have been synthesized by incorporating (S)- or (R)-1-phenylethylamine to both ends of the tetraindole scaffold. The oligoindoles fold into a helical conformation upon binding an anion by hydrogen bonds, which gives rise to an induced circular dichroism (CD) signal of large amplitude, implying the preferential formation of one helical isomer over another. Theoretical calculations suggest that the (P)-helix of the (S,S)-oligoindole 8a be more energetically stable than the corresponding (M)-helix.

Communication: Avoiding unbound anions in density functional calculations

Converged approximate density functional calculations usually do not bind anions due to large self-interaction error. But Hartree-Fock (HF) calculations have no such problem, producing negative HOMO energies. Thus, electron affinities can be calculated from density functional total energy differences using approximations such as PBE and B3LYP, evaluated on HF densities (for both anion and neutral). This recently proposed scheme is shown to work very well for molecules, better than the common practice of restricting the basis set except for cases such as CN, where the HF density is too inaccurate due to spin contamination.

Quantum time correlation functions from complex time Monte Carlo simulations: A maximum entropy approach

We present a way of combining real-time path integral Monte Carlo simulations with a maximum entropy numerical analytic continuation scheme in a new approach for calculating time correlation functions for finite temperature many body quantum systems. The real-time dynamics is expressed in the form of the symmetrized time correlation function, which is suitable for Monte Carlo methods, and several simulation techniques are presented for evaluating this function accurately up to moderate values of time. The symmetrized time correlation function is then analytically continued in combination with imaginary time data to obtain the real-time correlation function. We test this approach on several exactly solvable problems, including two one-dimensional systems, as well two cases of vibrational relaxation of a system coupled to a dissipative environment. The computed time correlation functions are in good agreement with exact results over several multiples of the thermal time βℏ, and exhibit a significant improve...

Quantum dynamics for a system coupled to slow baths: On-the-fly filtered propagator method

An on-the-fly filtered propagator functional path integral scheme is introduced as an efficient way of calculating the iterative dynamics of complex condensed systems. Time evolution of the reduced density matrix of a dissipative quantum system is evaluated iteratively by filtering the negligible propagator elements at each propagation step. This on-the-fly filtering along with the finiteness of the bath memory dramatically reduces the configuration space to be integrated without losing numerical accuracy of the results. The required computational storage space for the configuration and the weight of the survived path segments increases linearly with the bath memory length. Up to the bath memory time, it is found that a strikingly small fraction of the configurations survives the on-the-fly filtering process and the number of surviving configurations increases algebraically with the propagation time. At times longer than the bath memory time, the number of surviving configurations required for numerically...

Tensor propagator with weight-selected paths for quantum dissipative dynamics with long-memory kernels

Abstract The tensor multiplication scheme, which allows iterative evaluation of the path integral for quantum dissipative dynamics, is extended to processes involving long-memory kernels such as those occurring in low-frequency solvents. Rather than propagating the augmented reduced density tensor in full-dimensional space, we propose here including in the propagator tensor only those path segments which enter the path integral with appreciable weight. The appropriate path segments are selected according to their weights by importance sampling. Elimination of the vast majority of paths which make negligible contribution in the path integral results in dramatic reduction of the required storage.

Time‐dependent discrete variable representations for quantum wave packet propagation

We present an efficient method for exact wave function propagation with several degrees of freedom based on time‐dependent discrete variable representations (TD‐DVR) of the evolution operator. The key idea is to use basis sets that evolve in time according to appropriate reference Hamiltonians to construct TD‐DVR grids. The initial finite basis representation is chosen to include the initial wavefunction and thus the evolution under the bare zeroth order Hamiltonian is described at each time by a single DVR point. For this reason TD‐DVR grids offer optimal representations in time‐dependent calculations, allowing significant reduction of grid size and large time steps while requiring numerical effort that (for systems with several degrees of freedom) scales almost linearly with the total grid size. The method is readily applicable to systems described by time‐dependent Hamiltonians. TD‐DVR grids based on the time‐dependent self‐consistent field approximation are shown to be very useful in the study of intramolecular or collision dynamics.