Our condensed matter research is based on numerical simulation. This group consists of 9 members; one associate professor, two assistant professors, one ISSP research fellow, one postdocs, three students, and one office administrator.

Main targets are (1) material research using the numerical simulation and (2) development of the simulation scheme.

## Research Activity

**Material Simulation**

Our material simulation is based on the density functional theory (DFT) and the many-body perturbation theory, i.e., the GW + Bethe-Salpeter equation method.

#### Electronic structure from the many-body perturbation theory (the GW + Bethe-Salpeter equation method)

Noguchi has developed a code for the GW + Bethe-Salpeter calculation and has applied to novel cluster systems, CdSe clusters [1], C60 encapsulating a proton or an alkali metal atom [2], and the warped nanographene that contains 5-7 membered rings as a defect. The photo-absorption spectra are expected as an important means to identifying their structures.

Hirose and Shishido have applied the code to small molecules and nanographenes to characterize the many-body effect as well as limitation of the present many-body perturbation method.

[1] Y. Noguchi et al. (2012)

[2] Y. Noguchi et al. (2013)

[3] Y. Noguchi et al. (2015)

#### Structure of the solid-liquid interface and the reaction dynamics

This group has been applying a first-principles molecular dynamics method to the non-equilibrium solid-liquid interfaces. So far, we have done the simulation on the Volmer step of the hydrogen evolution reaction [1] and the full steps of the oxygen reduction reaction [2]. The former studied a Pt(111) surface only but the latter studied (111) surfaces of Pd, Ru, and Au as well. The accuracy check of these simulations has been done only partially, but it is emphasized that the simulations were done under a finite temperature and a certain bias conditions without assuming a particular mechanism. In this sense, they can be regarded as a computer experiment.

[1] M. Otani et al.(2008)

[2] Y. Okamoto et al. (2010)

We have also done Monte Carlo simulation using a lattice gas model that can reproduce the DFT total-energy and have studied the dominant adsorption site for the hydrogen atom [3]. The combined first-principles and the model simulations are particularly important for studying this complex system.

#### Firefly luciferin

Firefly luciferin is light-emitting compound of firely species. Akiyama group in ISSP has been studing a mechism for the light-emitting. Noguchi was inspired by this group and did a collaborative study on the photoabsorption spectra of this material as a first step towards the full understanding [1].

**Development of a computationa scheme**

#### Many-body perturbation method

Noguchi and Hirose have been developing a code for the GW + Bethe-Salpeter equation calculation. This method, based on the DFT and the many-body perturbation, can be applied to systems up to one hundred carbon atoms owing to the massively parallel supercomputers. This method has been applied to Fullerenes [1], nanographenes [2], and luciferins [3] to characterize their absorption spectra.

The code is based on the all-electron mixed basis scheme and is thus able to treat the core excitations. Noguchi [4] has applied to the x-ray absroption of typical molecules and has found that the code can reproduce the spectra within the error of 2-5 eV, significantly improving the TD-DFT calculations where the error is larger than 10 eV.

[1] Y. Noguchi et al. (2013)

[2] Y. Noguchi et al. (2015)

[3] Y. Noguchi et al. (2014)

[4] Y. Noguchi et al. (2015)

#### Accurate wavefunction theory

The wavefunction of an N-electron system is 3N dimensional anti-symmetric function. Because of the vast Hilbert space, it is an extremely challenging theme to determine the ground state wavefunction.

The wavefunction can be characterized by the high-rank antisymmetric tensor, and the key is in an efficient description of the tensor, which is not possible with the linear combination of the Slater determinants. Uemura [1] has found that it is possible to decompose the tensor into low-rank tensors and showed its effectiveness using a few small molecules, H2, He2 and LiH. Uemura and Kasamatsu are now studying an extension of this method for the delveoplement of an accurate-and-efficient electronic structure calculation.

#### Energy conversion at interfaces

This group has long been developing a method for simulating the electrode-electrlyte interface on the basis of the first-principles electronic structure calculation [1]. The goal is to fully sample the phase space under given finite temperature and chemical potential conditions, and towards the realization of the grand canonical ab initio MD we have been collaborating with researchers from Tohoku U., Osaka U, and national laboratories (AIST and NIMS).

[1] ESM

## Education

Sugino group participates in the physics department of the graduate school of the University of Tokyo. http://www.issp.u-tokyo.ac.jp/maincontents/education_en.html

Please visit Department of Physics for details.