Comparative study of first-principles approaches for effective Coulomb interaction strength Ueff between localized f-electrons: Lanthanide metals as an example.

As correlation strength has a key influence on the simulation of strongly correlated materials, many approaches have been proposed to obtain the parameter using first-principles calculations. However, a comparison of the different Coulomb strengths obtained using these approaches and an investigation of the mechanisms behind them are still needed. Taking lanthanide metals as an example, we research the factors that affect the effective Coulomb interaction strength, Ueff, by local screened Coulomb correction (LSCC), linear response (LR), and constrained random-phase approximation (cRPA) in the Vienna Ab initio Simulation Package. The Ueff LSCC value increases from 4.75 to 7.78 eV, Ueff LR is almost stable at about 6.0 eV (except for Eu, Er, and Yb), and Ueff cRPA shows a two-stage decreasing trend in both light and heavy lanthanides. To investigate these differences, we establish a scheme to analyze the coexistence and competition between the orbital localization and the screening effect. We find that LSCC and cRPA are dominated by the orbital localization and the screening effect, respectively, whereas LR shows the balance of the competition between the two factors. Additionally, the performance of these approaches is influenced by different starting points from the Perdew-Burke-Ernzerhof (PBE) and PBE + U, especially for cRPA. Our results provide useful knowledge for understanding the Ueff of lanthanide materials, and similar analyses can also be used in the research of other correlation strength simulation approaches.

Atomic force microscopy for revealing micro/nanoscale mechanics in tumor metastasis: from single cells to microenvironmental cues.

Mechanics are intrinsic properties which appears throughout the formation, development, and aging processes of biological systems. Mechanics have been shown to play important roles in regulating the development and metastasis of tumors, and understanding tumor mechanics has emerged as a promising way to reveal the underlying mechanisms guiding tumor behaviors. In particular, tumors are highly complex diseases associated with multifaceted factors, including alterations in cancerous cells, tissues, and organs as well as microenvironmental cues, indicating that investigating tumor mechanics on multiple levels is significantly helpful for comprehensively understanding the effects of mechanics on tumor progression. Recently, diverse techniques have been developed for probing the mechanics of tumors, among which atomic force microscopy (AFM) has appeared as an excellent platform enabling simultaneously characterizing the structures and mechanical properties of living biological systems ranging from individual molecules and cells to tissue samples with unprecedented spatiotemporal resolution, offering novel possibilities for understanding tumor physics and contributing much to the studies of cancer. In this review, we survey the recent progress that has been achieved with the use of AFM for revealing micro/nanoscale mechanics in tumor development and metastasis. Challenges and future progress are also discussed.

Local screened Coulomb correction approach to strongly correlated d-electron systems.

Materials with open-shell d or f-electrons are of great importance for their intriguing electronic, optical, and magnetic properties. Often termed as strongly correlated systems, they pose great challenges for first-principles studies based on density-functional theory (DFT) in the local density approximation or generalized gradient approximation (GGA). The DFT plus the Hubbard U correction (DFT + U) approach, which is widely used in first-principles studies of strongly correlated systems, depends on the local Coulomb interaction parameters (the Hubbard U and the Hund exchange J) that are often chosen empirically, which significantly limits its predictive capability. In this work, we propose a local screened Coulomb correction (LSCC) approach in which the on-site Coulomb interaction parameters are determined by the local electron density based on the Thomas-Fermi screening model in a system-dependent and self-consistent way. The LSCC approach is applied to several typical strongly correlated systems (MnO, FeO, CoO, NiO, ß-MnO2, K2CuF4, KCuF3, KNiF3, La2CuO4, NiF2, MnF2, KMnF3, K2NiF4, La2NiO4, and Sr2CuO2Cl2), and the results are compared to those obtained from the hybrid functional and GGA methods. We found that the LSCC method can provide an accurate description of electronic and magnetic properties of considered strongly correlated systems and its performance is less sensitive to the effective range of the local projection than the closely related DFT + U approach. Therefore, the LSCC approach provides a parameter-free first-principles approach to strongly correlated systems.

The local projection in the density functional theory plus U approach: A critical assessment.

Density-functional theory plus the Hubbard U correction (DFT + U) method is widely used in first-principles studies of strongly correlated systems, as it can give qualitatively (and sometimes, semi-quantitatively) correct description of energetic and structural properties of many strongly correlated systems with similar computational cost as local density approximation or generalized gradient approximation. On the other hand, the DFT + U approach is limited both theoretically and practically in several important aspects. In particular, the results of DFT + U often depend on the choice of local orbitals (the local projection) defining the subspace in which the Hubbard U correction is applied. In this work we have systematically investigated the issue of the local projection by considering typical transition metal oxides, ß-MnO2 and MnO, and comparing the results obtained from different implementations of DFT + U. We found that the choice of the local projection has significant effects on the DFT + U results, which are more significant for systems with stronger covalent bonding (e.g., MnO2) than those with more ionic bonding (e.g., MnO). These findings can help to clarify some confusion arising from the practical use of DFT + U and may also provide insights for the development of new first-principles approaches beyond DFT + U.

Nanoscale monitoring of drug actions on cell membrane using atomic force microscopy.

Knowledge of the nanoscale changes that take place in individual cells in response to a drug is useful for understanding the drug action. However, due to the lack of adequate techniques, such knowledge was scarce until the advent of atomic force microscopy (AFM), which is a multifunctional tool for investigating cellular behavior with nanometer resolution under near-physiological conditions. In the past decade, researchers have applied AFM to monitor the morphological and mechanical dynamics of individual cells following drug stimulation, yielding considerable novel insight into how the drug molecules affect an individual cell at the nanoscale. In this article we summarize the representative applications of AFM in characterization of drug actions on cell membrane, including topographic imaging, elasticity measurements, molecular interaction quantification, native membrane protein imaging and manipulation, etc. The challenges that are hampering the further development of AFM for studies of cellular activities are aslo discussed.

Theoretical studies of the work functions of Pd-based bimetallic surfaces.

Work functions of Pd-based bimetallic surfaces, including mainly M/Pd(111), Pd/M, and Pd/M/Pd(111) (M = 4d transition metals, Cu, Au, and Pt), are studied using density functional theory. We find that the work function of these bimetallic surfaces is significantly different from that of parent metals. Careful analysis based on Bader charges and electron density difference indicates that the variation of the work function in bimetallic surfaces can be mainly attributed to two factors: (1) charge transfer between the two different metals as a result of their different intrinsic electronegativity, and (2) the charge redistribution induced by chemical bonding between the top two layers. The first factor can be related to the contact potential, i.e., the work function difference between two metals in direct contact, and the second factor can be well characterized by the change in the charge spilling out into vacuum. We also find that the variation in the work functions of Pd/M/Pd(111) surfaces correlates very well with the variation of the d-band center of the surface Pd atom. The findings in this work can be used to provide general guidelines to design new bimetallic surfaces with desired electronic properties.

Doubly Screened Coulomb Correction Approach for Strongly Correlated Systems.

Strongly correlated systems containing d/f electrons present a challenge to conventional density functional theory such as the local density approximation or generalized gradient approximation. We developed a doubly screened Coulomb correction (DSCC) approach to perform on-site Coulomb interaction correction for strongly correlated materials. The on-site Coulomb interaction between localized d/f electrons is self-consistently determined from a model dielectric function that includes both the static dielectric and Thomas-Fermi screening. We applied DSCC to simulate the electronic and magnetic properties of typical 3d, 4f, and 5f strongly correlated systems. The accuracy of DSCC is comparable to that of hybrid functionals but an order of magnitude faster. In addition, DSCC can reflect the difference in the Coulomb interaction between metallic and insulating situations, similar to the popular but computationally expensive constrained random phase approximation approach. This feature suggests that DSCC is also a promising method for simulating Coulomb interaction parameters.

Drift compensation and faulty display correction in robotic nano manipulation.

Random drift and faulty visual display are the main problems in Atomic Force Microscopy (AFM) based robotic nanomanipulation. As far as we know, there are no effective methods available to solve these problems. In this paper, an On-line Sensing and Display (OSD) method is proposed to solve these problems. The OSD method mainly includes two subprocesses: Local-Scan-Before-Manipulation (LSBM) and Local-Scan-After-Manipulation (LSAM). During manipulation, LSBM and LSAM are on-line performed for random drift compensation and faulty visual display correction respectively. Through this way, the bad influence aroused from random drift and faulty visual display can be eliminated in real time. The visual feedback keeps consistent with the true environment changes during the process of manipulation, which makes several operations being finished without an image scan in between. Experiments show the increased effectiveness and efficiency of AFM based nanomanipulation.

Doubly Screened Hybrid Functional: An Accurate First-Principles Approach for Both Narrow- and Wide-Gap Semiconductors.

First-principles prediction of electronic band structures of materials is crucial for rational material design, especially in solar-energy-related materials science. Hybrid functionals that mix the Hartree-Fock exact exchange with local or semilocal density functional approximations have proven to be accurate and efficient alternatives to more sophisticated Green's function-based many-body perturbation theory. The optimal fraction of the exact exchange, previously often treated as an empirical parameter, is closely related to the screening strength of the system under study. From a physical point of view, the screening has two extreme forms: the dielectric screening [1/ϵM] that is dominant in wide-gap materials and the Thomas-Fermi metallic screening [exp(-ζ r) ] that is important in narrow-gap semiconductors. In this work, we have systematically investigated the performances of a nonempirical doubly screened hybrid (DSH) functional that considers both screening mechanisms and found that it excels all other existing hybrid functionals and describes the band gaps of narrow-, medium-, and wide-gap insulating systems with comparably good performances.

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Using the winter wheat cultivar Tainong 18 as the experimental material, we analyzed yield stability from 2012 to 2016 under three different treatments: T1(following typical local field management practices), T2(high-yield: high nitrogen and water were supplied to foster high grain yield), and T3(high-yield, high-efficiency: optimized field management including increasing plant density, reducing nitrogen input and delaying of the sowing date). Yield related phenotypic traits, including the number of ears on the main stem and tillers, leaf area index (LAI), photosynthetically active radiation (PAR) interception, dry matter accumulation and distribution, as well as grain yield, were analyzed over four seasons to determine their relationships with annual radiation, accumulated temperature and precipitation. We then determined grain yield stability for each of the three treatments. The amount and distribution of radiation, accumulated temperature, and precipitation varied greatly within each season. The ears on the main stem represented 38.9%, 58.7%, and 66.9% of the total ears, respectively, for wheat grown in the T1, T2 and T3 treatments, indicating that T1 ears originated mainly from the tillers, T2 ears from both the main stem and the tillers, and T3 ears from the main stem. The T2 and T1 treatments produced the highest and lowest amount of dry matter and grain yield, respectively. Although having relatively lower dry matter accumulation at maturity compared with T2, T3 led to higher grain yield due to high LAI, high PAR interception and utilization, high net canopy photosynthetic rate from booting (especially from 14 days after anthesis) to maturity and a higher harvest index. Among the three treatments, T3 resulted in the lowest annual range, standard deviation, and coefficient of variation for LAI, PAR interception, and dry matter accumulation. Thus, grain yield was most stable in wheat grown in the T3 treatment mainly due to stability in biological production during all four seasons.

[Effects of cultivation patterns on the radiation use and grain yield of winter wheat].

Taking winter wheat cultivar 'Tainong 18' as test material, this paper set three treatments, local farmer's traditional cultivation pattern (FP), super high yield pattern (SH) and high yield high efficiency pattern ( HH) to investigate the effects of cultivation patterns on the intercepted photosynthetically active radiation (IPAR), PAR use efficiency (RUE), dry matter (DM) accumulation, harvest index (HI), grain yield and fertilizers' partial factor productivity (PFP) in 2012-2013. The results showed that IPAR, RUE and DM accumulation of the total growth stage and grain yield under SH pattern were significantly higher than those under FP pattern. IPAR of the total growth stage under HH pattern was lower than that under FP pattern, but RUE, DM accumulation and HI were significantly higher than that under FP pattern, so grain yield was higher than that under FP pattern. The grain yields under HH pattern were respectively decreased by 3.8% and 2.8% under high and low fertility levels compared that under SH pattern, while the PFP of N, P and K under HH pattern were averagely 26.4%, 68.5% and 92.6% higher than those under SH pattern, respectively. In conclusion, HH pattern, with the characteristics of 'reducing fertilizer', 'increasing planting density' and 'delaying sowing date', was the recommended cultivation pattern under the condition similar to this experiment balancing the grain yield, radiation use and fertilizer use.