DP Compress: A Model Compression Scheme for Generating Efficient Deep Potential Models.

Machine-learning-based interatomic potential energy surface (PES) models are revolutionizing the field of molecular modeling. However, although much faster than electronic structure schemes, these models suffer from costly computations via deep neural networks to predict the energy and atomic forces, resulting in lower running efficiency as compared to the typical empirical force fields. Herein, we report a model compression scheme for boosting the performance of the Deep Potential (DP) model, a deep learning-based PES model. This scheme, we call DP Compress, is an efficient postprocessing step after the training of DP models (DP Train). DP Compress combines several DP-specific compression techniques, which typically speed up DP-based molecular dynamics simulations by an order of magnitude faster and consume an order of magnitude less memory. We demonstrate that DP Compress is sufficiently accurate by testing a variety of physical properties of Cu, H2O, and Al-Cu-Mg systems. DP Compress applies to both CPU and GPU machines and is publicly available online.

Pauli Repulsion Enhances Mobility of Ultraconfined Water.

Water is ubiquitous on Earth and dominates chemical and biological processes in daily life. However, how water behaves under some critical conditions is not fully understood. In this paper, we employed quantum first-principles calculations and dynamics simulations to reveal the unexpectedly high mobility of water molecules in ultraconfined spaces. The water molecules rotated more freely in the (4, 4) carbon nanotube than in the (5, 5) carbon nanotube, which is induced by the Pauli repulsion from the wall of the narrower channel when reducing the size of the channel from general confinement to ultraconfinement. Moreover, this quantum effect facilitates the transport of water molecules into the space within their van der Waals diameter easily, which is in contrast to the general understanding. Thus, the conventional concept that the tighter the confined space, the more difficult the motion of the confined object is not always correct. This quantum-induced enhancement of water mobility by Pauli repulsion calls us to pay more attention to the existence and the function of water in neglected ultraconfined spaces (e.g., cells and the Earth's crust) in the future.

The nature of small molecules adsorbed on defective carbon nanotubes.

In this work, we perform a comprehensive theoretical study on adsorption of representative 10-electron molecules H2O, CH4 and NH3 onto defective single-walled carbon nanotubes. Results of adsorption energy and charge transfer reveal the existence of both chemical adsorption (CA) and physical adsorption (PA). While PA processes are common for all molecules, CA could be further achieved by the polar molecule NH3, whose lone-pair electrons makes it easier to be bonded with the defective nanotube. Our systematic work could contribute to the understanding on intermolecular interactions and the design of future molecular detectors.

Binding for endohedral-metallofullerene superatoms induced by magnetic coupling.

To design new materials based on artificial superatoms, clarifying their involved interaction is particularly important. In this study, we discuss first-principle calculations to show that the interaction between endohedral metallofullerenes (EMFs) of U@C28 can lead to different chemical and physical adsorption structures. Especially, these structures are derived from different magnetic coupling resonances, and they can transform by changing the distance between U@C28 superatoms. These findings will promote the future development for bottom-up assembling of new functional materials and even devices.

Effect of confinement on water rotation via quantum tunnelling.

Water exhibits different behaviors in confined space compared to free space, which is critical for desalination, biosensing, and many potential applications. Recent studies indicated that quantum tunnelling plays an important role in the orientation of H2O molecules and the H-bond network of water clusters, but whether this effect is important in confined space remains elusive. Here, we studied the quantum tunnelling effect of water dimers in carbon nanotubes with different sizes by first-principles calculations. Our results show that though this effect may be negligible at room temperature, it becomes dominant at low temperatures up to ∼100 Kelvin. In particular, with the injection of a small amount of energy to excite a specific vibrational mode, the tunnelling rotation effect can be significantly enhanced, which provides a new strategy to tune the H-bond network of confined water.

Engineering a red emission of copper nanocluster self-assembly architectures by employing aromatic thiols as capping ligands.

Luminescent Cu nanoclusters (NCs) are potential phosphors for illumination and display, but the difficulty in achieving full-color emission greatly limits practical applications. On the basis of our previous success in preparing Cu NC self-assembly architectures with blue-green and yellow emission, in this work, Cu NC self-assembly architectures with strong red emission are prepared by replacing alkylthiol ligands with aromatic thiols. The introduction of aromatic ligands is able to influence the ligand-to-metal charge transfer and/or ligand-to-metal-metal charge transfer, thus permitting the tuning of the emission color and enhancing of the emission intensity. The emission color can be tuned from yellow to dark red by choosing the aromatic ligands with different conjugation capabilities, and the photoluminescence quantum yield is up to 15.6%. Achieving full-color emission Cu NC self-assembly architectures allows the fabrication of Cu NC-based white light-emitting diodes.

Unusual spin-polarized electron state in fullerene induced by carbon adatom defect.

First-principles calculations show that a carbon adatom defect at the Def[5, 6] site on the surface of C60 can produce a more stable spin-polarized singlet electronic state instead of a magnetic triplet state. This is clearly different from the cases of graphene and nanotubes. The mechanism results from the electron population of the adatom, which produces antiferromagnetic coupling around the C60 cage and the adatom itself. Our calculations show the same phenomenon occurs in other IPR fullerenes, such as C70 and C80. These findings extend the understanding of the magnetic origin of pure carbon structures and are valuable for research related to the spin polarization of carbon systems.

Molecular orbital analysis of the hydrogen bonded water dimer.

As an essential interaction in nature, hydrogen bonding plays a crucial role in many material formations and biological processes, requiring deeper understanding. Here, using density functional theory and post-Hartree-Fock methods, we reveal two hydrogen bonding molecular orbitals crossing the hydrogen-bond's O and H atoms in the water dimer. Energy decomposition analysis also shows a non-negligible contribution of the induction term. Our finding sheds light on the essential understanding of hydrogen bonding in ice, liquid water, functional materials and biological systems.

Charging-induced asymmetric spin distribution in an asymmetric (9,0) carbon nanotube.

Asymmetry in the electronic structure of low-dimensional carbon nanomaterials is important for designing molecular devices for functions such as directional transport and magnetic switching. In this paper, we use density functional theory to achieve an asymmetric spin distribution in a typical (9,0) carbon nanotube (CNT) by capping the CNT with a fullerene hemisphere at one end and saturating the dangling bonds with hydrogen atoms at the other end. The asymmetric structure facilitates obvious asymmetry in the spin distribution along the tube axis direction, with the maximum difference between the ends reaching 1.6 e Å(-1). More interestingly, the heterogeneity of the spin distribution can be controlled by charging the system. Increasing or decreasing the charge by 2e can reduce the maximum difference in the linear spin density along the tube axis to approximately 0.68 e Å(-1) without changing the proportion of the total electron distribution. Further analyses of the electron density difference and the density of states reveal the loss and gain of charge and the participation of atomic orbitals at both ends. Our study characterizes the asymmetric spin distribution in a typical asymmetric carbon system and its correlation with charge at the atomic level. The results provide a strategy for controlling the spin distribution for functional molecular devices through a simple charge adjustment.

Depolymerization of Free-Radical Polymers with Spin Migrations.

The mechanism of depolymerization is one of the most essential issues in chemical engineering and materials science. In this work, we investigate the depolymerization reactions of three typical free-radical poly(alpha-methylstyrene) tetramers by using first-principles density functional theory. The calculated results show that these reactions all need to overcome the energy barriers in the range of 0.58 to 0.77 eV, and that breaking the C-C bond at the chain end leads to the dissociation of alpha-methylstyrene monomers from the polymers. Electronic-structure analysis indicates that the reactions occur easily at the CR3 unsaturated end, and that the frontier molecular orbitals that participate in the reactions are mainly localized at the unsaturated ends. Meanwhile, spin population analysis presents the unique net spin-transfer process in free-radical depolymerization reactions. We hope the current findings can contribute to understanding the free-radical depolymerization mechanism and help guide future experiments.

U@C28: the electronic structure induced by the 32-electron principle.

First principles calculations show that the neutral U@C28 has a (cage)(2) ground state with Td symmetry instead of the long believed (5f)(1)(cage)(1) ground state with D2 symmetry. Its 34 valence electrons preferentially obey the 32-electron principle which fills all the s-, p-, d-, and f-type valence shells of the uranium atom. The remaining two valence electrons cannot break the electronic configuration and thus are located on the cage.

Slippage in stacking of graphene nanofragments induced by spin polarization.

Spin polarization and stacking are interesting effects in complex molecular systems and are both presented in graphene-based materials. Their possible combination may provide a new perspective in understanding the intermolecular force. The nanoscale graphene structures with zigzag edges could possess spin-polarized ground states. However, the mechanical effect of spin polarization in stacking of graphene nanofragments is not clear. Here we demonstrate the displacement between two stacked rhombic graphene nanofragments induced by spin polarization, using first-principles density-functional methods. We found that, in stacking of two rhombic graphene nanofragments, a spin-polarized stacked conformation with zero total spin is energetically more favorable than the closed-shell stacking. The spin-polarized conformation gives a further horizontal interlayer displacement within 1 angstrom compared with the closed-shell structure. This result highlights that, besides the well-known phenomenologically interpreted van der Waals forces, a specific mechanism dependent on the monomeric spin polarization may lead to obvious mechanical effects in some intermolecular interactions.

Electronic delocalization in small water rings.

Water clusters are known to form through hydrogen bonding. However, this study shows that the formation of small water clusters such as (H2O)n with n = 3 or 4 involves strong electron delocalization. Our first-principles calculations reveal that the electron delocalization originates from both the H and O atomic orbitals and extends to the ring center, enriching the bonding characteristics of water clusters.