Jer-Lai Kuo

Metallic VS2 Monolayer Polytypes as Potential Sodium-Ion Battery Anode via ab Initio Random Structure Searching.

We systematically investigated the potential of single-layer VS2 polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2 can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assumed. With reasonably high specific energy capacity (232.91 and 116.45 mAh/g for 1H and 1T phases, respectively) and open-circuit voltage (1.30 and 1.42 V for 1H and 1T phases, respectively), VS2 is a promising alternative material for Na-ion battery anodes with great structural sturdiness. Finally, we have shown the capability of the ab initio random structure searching in the assessment of potential materials for energy storage applications.

Features in Vibrational Spectra Induced by Ar-Tagging for H3O+Arm, m = 0–3

Understanding the spectral features for solvated hydronium has been hindered due to the strong and complex vibrational couplings that lead to broad bands in the aqueous phase. In this work, utilizing ab initio vibrational calculations, we determine how the vibrational couplings induced by the Ar microsolvation in H3O(+)Arm m = 0-3 affect the observed spectra. With theoretical peak intensities and peak positions, we assign the experimental spectra. We also show that an increase in the number of Ar atoms results in an anticooperative blue shifting in the Ar-tagged OH stretching bands. This change in peak position of the OH stretching fundamental modulates the Fermi resonance with the bending overtone. This is observed as a distinct doublet feature at 3200 cm(-1) with varying intensities for H3O(+)Ar2 and H3O(+)Ar3. The coupling between the in-plane rotation of the hydronium and the bending modes of H3O(+) leads to the existence of a strong association bands around 1900 cm(-1).

Structural and Electronic Properties of Monolayer 1T-MoS2 Phase, and Its Interaction with Water Adsorbed on Perfect, Single S-Vacated and MoS2-Unit-Vacated Surface: Density Functional Theory Calculations

Using density functional theory calculations with van der Waals dispersion corrections, we studied the structural and electronic properties of monolayer 1T-MoS2, and how it interacts with water on perfect and vacancy-doped cases, which include single sulfur and MoS2-unit vacancies. We first optimized and calculated its structural parameters and electronic properties and confirmed earlier reports suggesting its metastable state and metallic electronic structure. Results show that adsorption sites on perfect surface bind water weakly while the opposite is true for vacancy sites adsorption. Climbing nudged elastic band calculations revealed that sulfur vacancy site could favorably dissociate water and evolve hydrogen with moderate energy cost, while significantly high energies are required for the same processes on MoS2 unit vacancy. The results are important in supplementing efforts to find clean and renewable energy sources.

An ab initio anharmonic approach to study vibrational spectra of small ammonia clusters

Fermi resonance between the N-H stretching (ν1 and ν3) and the overtone of N-H bending (2ν4) in ammonia has hindered the interpretation and assignments of experimental spectra of small ammonia clusters. In this work, we carried out anharmonic vibrational calculations using MP2/aug-cc-pVDZ to examine the vibrational spectra of (NH3)n=1-5 with a focus on the size evolution. The enhancement of hydrogen bond strength due to cooperative effects will cause ν1 and ν4 to red-shift and blue-shift, respectively, when the size of the cluster increases. Our calculations show that the energy order of fundamental of ν1 and overtone of ν4 is reversed between n = 3 and n = 4. Therefore, while the resultant mixed levels do not show remarkable shifts in their peak positions, the main identity of these mixed levels changes and this causes significant re-distribution of their intensities. Furthermore, our ab initio anharmonic calculation scheme can directly evaluate the coupling strength between different N-H stretching and overtone of N-H bending without any experimental parameters, thus leading us to a simpler picture to understand the Fermi resonance in (NH3)n.

Tunable Gravimetric and Volumetric Hydrogen Storage Capacities in Polyhedral Oligomeric Silsesquioxane Frameworks

We study the hydrogen adsorption in porous frameworks composed of silsesquioxane cages linked via boron substituted aromatic structures by first-principles modeling. Such polyhedral oligomeric silsesquioxane (POSS) frameworks can be further modified by decorating them with metal atoms binding to the ring structures of the linkers. We have considered Sc- and Ti-doped frameworks which bind H2 via so-called Kubas interaction between hydrogen molecules and transition metal atoms. It will be demonstrated that the maximum H2 gravimetric capacity can be improved to more than 7.5 wt % by using longer linkers with more ring structures. However, the maximum H2 volumetric capacity can be tuned to more than 70 g/L by varying the size of silsesquioxane cages. We are optimistic that by varying the building blocks, POSS frameworks can be modified to meet the targets for the gravimetric and volumetric capacities set by the U.S. Department of Energy.

Tuning the vibrational coupling of H3O+ by changing its solvation environment

This study demonstrates how the intermode coupling in the hydronium ion (H3O+) is modulated by the composition of the first solvation shell. A series of rare gas solvated hydronium ions (H3O+Rg3, where Rg = Ne, Ar, Kr, and Xe) is examined via reduced-dimensional anharmonic vibrational (RDAV) ab initio calculations. We considered six key vibrational normal modes, namely: a hindered rotation, two H-O-H bends, and three O-H stretches. Between the O-H stretches and the H-O-H bends, the first is more sensitive to solvation strength. Our calculations revealed that the Fermi resonance between the first overtones of O-H bends and the fundamentals of O-H stretches led to complex spectral features from 3000 to 3500 cm-1. Such an interaction is not only sensitive to the type of rare gas messengers surrounding the H3O+ ion, it also exhibits an anomalous H → D isotope effect. Although it is accepted that visible combination tones (∼1900 cm-1) arise from the complex coupling between the hindered rotation and the H-O-H bends, the origin of their intensities is not yet clearly understood. We found that the intensity of these combination tones could be much stronger than their fundamental H-O-H bends. Within our theoretical framework, we tracked the combination tones intensity back to the asymmetric O-H stretches. This simple notion of intensity borrowing is confirmed by examining eight complexes (H3O+·Rg3 and D3O+·Rg3) with spectral features awaiting experimental confirmations.

Proton Quantum Confinement on Symmetric Dimers of Ammonia and Lower Amine Homologs

Behavior of shared proton in symmetric dimers of ammonia and lower amine homologs were studied by several theoretical methods. Corresponding optimized structures by density functional theory show an intuitive hypsochromic shift as the degree of methylation is enhanced. Inclusion of nuclear quantum effect, however, changes the whole picture. It was found out that the fundamental vibrational transition corresponding to the shared proton’s stretching motion, νsp is counter intuitive. Based from these calculations, there is a bathochromic shift from ammonia to trimethylamine. These ramifications do clearly indicate that proton is a quantum object. Furthermore, spectroscopic features for the stretching modes of the shared proton and H-bond donor-acceptor atoms were proposed.

A LAMMPS implementation of volume–temperature replica exchange molecular dynamics

Abstract A driver module for executing volume–temperature replica exchange molecular dynamics (VTREMD) was developed for the LAMMPS package. As a patch code, the VTREMD module performs classical molecular dynamics (MD) with Monte Carlo (MC) decisions between MD runs. The goal of inserting the MC step was to increase the breadth of sampled configurational space. In this method, states receive better sampling by making temperature or density swaps with their neighboring states. As an accelerated sampling method, VTREMD is particularly useful to explore states at low temperatures, where systems are easily trapped in local potential wells. As functional examples, TIP4P/Ew and TIP4P/2005 water models were analyzed using VTREMD. The phase diagram in this study covered the deeply supercooled regime, and this test served as a suitable demonstration of the usefulness of VTREMD in overcoming the slow dynamics problem. To facilitate using the current code, attention was also paid on how to optimize the exchange efficiency by using grid allocation. VTREMD was useful for studying systems with rough energy landscapes, such as those with numerous local minima or multiple characteristic time scales. Program summary Program title: vttemper Catalogue identifier: AEVB_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEVB_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 49706 No. of bytes in distributed program, including test data, etc.: 1249424 Distribution format: tar.gz Programming language: C++/MPI. Computer: Tested on Intel X86 and AMD64 architectures. Should be operational on any computer with a C++ compiler. Operating system: Tested on Linux. Should run on any platform with C++ and MPI library. Has the code been vectorized or parallelized?: Yes. 1 to N processors may be used. RAM: Depends on the system size and how the program is partitioned. Classification: 16.13. External routines: LAMMPS ( http://lammps.sandia.gov ) Nature of problem: The code implements volume–temperature replica exchange molecular dynamics for LAMMPS. Each replica has its own temperature and density. A two-dimensional temperature–density grid is constructed. Solution method: Extending the ability of the existing temper command in LAMMPS to handle replicas of two state variables. Restrictions: The code is based on LAMMPS. A well-designed grid on temperature and density is required. Running time: Running time depends on the system size and the complexity of the problem. Using 8 processors the test run takes approximately 1 minute.