Jun-Ichi Iwata

Real-space, real-time method for the dielectric function

We present an algorithm to calculate the linear response of periodic systems in the time-dependent density functional theory, using a real-space representation of the electron wave functions and calculating the dynamics in real time. The real-space formulation increases the efficiency for calculating the interaction, and the real-time treatment decreases storage requirements and allows the entire frequency-dependent dielectric function to be calculated at once. We give as examples the dielectric functions of a simple metal, lithium, and an elemental insulator, diamond.

A massively-parallel electronic-structure calculations based on real-space density functional theory

Abstract Based on the real-space finite-difference method, we have developed a first-principles density functional program that efficiently performs large-scale calculations on massively-parallel computers. In addition to efficient parallel implementation, we also implemented several computational improvements, substantially reducing the computational costs of O ( N 3 ) operations such as the Gram–Schmidt procedure and subspace diagonalization. Using the program on a massively-parallel computer cluster with a theoretical peak performance of several TFLOPS, we perform electronic-structure calculations for a system consisting of over 10,000 Si atoms, and obtain a self-consistent electronic-structure in a few hundred hours. We analyze in detail the costs of the program in terms of computation and of inter-node communications to clarify the efficiency, the applicability, and the possibility for further improvements.

Absence of Dirac Electrons in Silicene on Ag (111) Surfaces

We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene/Ag(111), exhibiting a variety of images of scanning tunneling microscopy. We also find that Dirac electrons are absent near Fermi level in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. This is the first theoretical investigation that clarify the absence of Dirac electrons in silicene due to the strong interaction with substrates. We instead propose that either BN substrate or hydrogen-processed Si surface is a good candidate to preserve Dirac electrons in silicene.

First-principles calculations of electron states of a silicon nanowire with 100,000 atoms on the K computer

Real space DFT (RSDFT) is a simulation technique most suitable for massively-parallel architectures to perform first-principles electronic-structure calculations based on density functional theory. We here report unprecedented simulations on the electron states of silicon nanowires with up to 107,292 atoms carried out during the initial performance evaluation phase of the K computer being developed at RIKEN. The RSDFT code has been parallelized and optimized so as to make effective use of the various capabilities of the K computer. Simulation results for the self-consistent electron states of a silicon nanowire with 10,000 atoms were obtained in a run lasting about 24 hours and using 6,144 cores of the K computer. A 3.08 peta-flops sustained performance was measured for one iteration of the SCF calculation in a 107,292-atom Si nanowire calculation using 442,368 cores, which is 43.63% of the peak performance of 7.07 peta-flops.

Real-space computation of dynamic hyperpolarizabilities

A real-space method is developed to calculate molecular hyperpolarizabilities in the time-dependent density functional theory. The method is based on the response function formalism which was developed by Senatore and Subbaswamy for the third harmonic generation of rare-gas atoms [Phys. Rev. A 35, 2440 (1987)]. The response equations are discretized in real space employing a uniform grid representation in the three-dimensional Cartesian coordinate, and are solved with iterative methods such as conjugate-gradient and conjugate-residual methods. The method works efficiently for both small and large molecules, and for any nonlinear optical processes up to third order. The spatial convergence of the calculation can be examined with two intuitive parameters, the grid spacing and the spatial box size. Applications of our method are presented for rare-gas atoms and molecules, N2, H2O, C2H4, C6H6, and C60. Our results agree well with other calculations employing basis functions except for a slight deviation in a ...

First-principles calculation of the electron dynamics in crystalline SiO2

We present a first-principles description for electron dynamics in crystalline SiO(2) induced by an optical field in both weak and intense regimes. We rely upon the time-dependent density-functional theory with the adiabatic local-density approximation, and a real-space and real-time method is employed to solve the time-dependent Kohn-Sham equation. The response calculation to a weak field provides us with information on the dielectric function, while the response to an intense field shows the optical dielectric breakdown. We discuss the critical threshold for the dielectric breakdown of crystalline SiO(2), in comparison with the results for diamond.

Solving the RPA Eigenvalue Equation in Real-Space

We present a computational method to solve the RPA eigenvalue equation employing a uniform grid representation in three-dimensional Cartesian coordinates. The conjugate gradient method is used for this purpose as an iterative method for a generalized eigenvalue problem. No construction of unoccupied orbitals is required in the procedure. We expect this method to be useful for systems lacking spatial symmetry to calculate accurate eigenvalues and transition matrix elements of a few low-lying excitations. Some applications are presented to demonstrate the feasibility of the method, considering the simplified mean-field model as an example of a nuclear physics system and the electronic excitations in molecules with time-dependent density functional theory as an example of an electronic system.

Performance evaluation of ultra-large-scale first-principles electronic structure calculation code on the K computer

Silicon nanowires are potentially useful in next-generation field-effect transistors, and it is important to clarify the electron states of silicon nanowires to know the behavior of new devices. Computer simulations are promising tools for calculating electron states. Real-space density functional theory (RSDFT) code performs first-principles electronic structure calculations. To obtain higher performance, we applied various optimization techniques to the code: multi-level parallelization, load balance management, sub-mesh/torus allocation, and a message-passing interface library tuned for the K computer. We measured and evaluated the performance of the modified RSDFT code on the K computer. A 5.48 petaflops (PFLOPS) sustained performance was measured for an iteration of a self-consistent field calculation for a 107,292-atom Si nanowire simulation using 82,944 compute nodes, which is 51.67% of the K computer’s peak performance of 10.62 PFLOPS. This scale of simulation enables analysis of the behavior of a silicon nanowire with a diameter of 10–20 nm.

Nonadiabatic generation of coherent phonons.

The time-dependent density functional theory (TDDFT) is the leading computationally feasible theory to treat excitations by strong electromagnetic fields. Here the theory is applied to coherent optical phonon generation produced by intense laser pulses. We examine the process in the crystalline semimetal antimony (Sb), where nonadiabatic coupling is very important. This material is of particular interest because it exhibits strong phonon coupling and optical phonons of different symmetries can be observed. The TDDFT is able to account for a number of qualitative features of the observed coherent phonons, despite its unsatisfactory performance on reproducing the observed dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent phonon generation is also examined and compared with the TDDFT calculations.

Si nanowire FET and its modeling

Because of its ability to effectively suppress off-leakage current with its gate-around configuration, the Si nanowire FET is considered to be the ultimate structure for ultra-small CMOS devices to the extent that the devices would be approaching their downsized limits. Recently, several experimental studies of Si nanowire FETs with on-currents much larger than those of planar MOSFETs have been published. Consequently, Si nanowire FETs are now gaining significant attention as the most promising candidate for mainstream CMOS devices in the 2020s. To enable the introduction of the Si nanowire FETs into integrated circuits, good compact models, which circuit designers can easily handle, are essential. However, it is a very challenging task to establish such a compact model, because the ID–VD characteristics of Si nanowire FETs are affected by the band structure of the nanowire, which is very sensitive to the nanowire diameter, cross-sectional shape, crystal orientation, mechanical stress and interface states. In this paper, the recent status of research on Si nanowire FETs in experimental and theoretical studies is described.