Theoretical research and material design by firstprinciples electronic structure theory

Much attention has been focused on material designs with revealing and controlling the physical properties of each material by calculating its electronic structure and timedependent behavior.
Macroscopic behavior of materials can be understood by classical mechanics.
However, we need to consider an effect of quantum mechanics to understand microscopic (less than nanoscale order) behavior.
The target of our laboratory is theoretical research and material design by firstprinciples electronic structure theory.
Firstprinciples electronic structure theory is the one that solves Schrödinger equation by using only a spatial position of nuclei without any empirical parameters and determines the electronic structure of realistic materials.
Firstprinciples electronic structure theory is almost a unique method which gives a quantitative prediction of electronic structure and physical properties on realistic materials without any empirical parameters,
in contrast to model calculations with parameters determined by experiments.
We will establish the foundation and the methodology for electronic structure calculations in the following two aspects;


(1) Large Scale Electronic Structure Calculations 
Establishing the quantum mechanical molecular dynamics simulation method and the technology of process simulator for nanoscale systems of semiconductors and metals with from ten thousands to ten millions atoms.


(2) Beyond LDA: Extension of the DFT to e.g. LDA+U, GW, LDA+DMFT and its Application to Strongly Correlated Electron Systems 
Developping the novel method of the first principle electronic structure calculations with combination of oneelectron band theory and manyelectron theory. 


Research Projects supported by external organizations 
(1) CRESTJST project
(In "Establishment of Next Generation Integration Simulation Technology")
"Novel Methodology of Electronic Structure Calculations by Combining Several Different Aspects" 

(2) GrantinAid for Scientific Research on Priority Area
"Development of New Quantum Simulators and Quantum Design" 

(3) ELSES(Extra Large Scale Electronic Structure calculation) 

Term: 
Firstprinciples electronic structure theory
The manybody Schrödinger equation should be solved in order to treat manyelectron systems with quantummechanical approach.
However, an approximate form of the manybody Schrödinger equation should be solved since we are not able to solve it exactly during finite time.
Then, we assume that each electron moves in an averaged potential formed by all other electrons and nuclei. According to this assumption, the electronic structure of materials is determined without using any empirical parameters. This method is called "Firstprinciples electronic structure theory".
The local density approximation (LDA) based on the density functional theory (DFT) is the most prevalent method among the firstprinciples electronic structure calculations.
The LDA is based on the variational principles for functional of the electron density.
Stable structures and dynamical process of materials are also calculated by using ionic forces in each time calculated directly from Firstprinciples electronic structure theory.
This is called "Firstprinciples molecular dynamics".


Strongly correlated electron systems 
Strongly correlated electron systems are systems which have strong Coulomb interactions comparable to electronhopping integrals.
Typical examples of a strongly correlated electron system are transition metal oxides.
Much attention has been focused on strongly correlated electron systems, since these systems show anomalous physical properties such as various spin, charge and orbital order, metalinsulator transition, Colossal MagnetoResistance (CMR), Hightemperature superconductivity and so on.
The valence orbitals in strongly correlated electron systems are partially filled and well localized 3d or 4f orbitals and hence play important roles.
These various physical properties in strongly correlated electron systems have been extensively used in recent development of device designs.
The local density approximation (LDA) based on the density functional theory (DFT) is a quit successful for electronic structure calculation of many real materials with weakly correlated materials. However, the LDA is hardly applicable to strongly correlated electron systems, since the LDA oneelectron potential is orbital independent and hence takes into account the Coulomb interaction as an averaged term. In particular, LDA overestimates the width of 3d bands and underestimates the band gap for strongly correlated electron systems.
To understand the physical phenomena on strongly correlated electron systems, more sophisticated method with dynamical electron correlation effects are needed.
