Research

An atomic- and molecular-level understanding of drug actions and the mechanisms of a variety of chemical reactions will provide insight for developing new drugs and materials. Although a number of diverse experimental methods have been developed, it still remains difficult to investigate the state of complex molecules and to follow chemical reactions in detail. Therefore, a theoretical molecular science that can predict the properties and functions of matter at the atomic and molecular levels by means of molecular theoretical calculations is keenly awaited as a replacement for experiment. Theoretical molecular science has recently made great strides due to progress in molecular theory and computer development. However, it is still unsatisfactory for practical applications. Consequently, our main goal is to realize an updated theoretical molecular science by developing a molecular theory and calculation methods to handle large complex molecules with high precision under a variety of conditions. To achieve our aim, we have so far developed several methods of calculation. Examples include a way for resolving a significant problem facing conventional methods of calculation, in which the calculation volume increases dramatically when dealing with larger molecules; a way for improving the precision of calculations in molecular simulations; and a way for high-precision calculation of the properties of molecules containing heavy atoms such as metal atoms.

Quantum chemistry software comprises immensely useful tools in material and biological science research. Widely diverse programs have been developed in Western countries as Japan has lagged. In fact, only a few programs have been developed in Japan. The mission of our research team is to provide K computer users with a high-performance software for quantum molecular simulation. In the early stage of the K computer project, no quantum chemistry software was available for general purpose and massively parallel computation on the K computer because not every program was designed for use on it. Therefore, we have chosen to develop a new comprehensive ab initio quantum chemistry software locally: NTChem. NTChem is completely new software that implements not only standard quantum chemistry approaches, but also original and improved theoretical methods that we have developed in our research work. The main features of the current version, NTChem2013, are the following:

1. Electronic structure calculation of the ground state of atoms and molecules based on Hartree–Fock (HF) and density functional theory (DFT) methods.
2. Linear-scaling or low-scaling DFT: Gaussian and finite-element Coulomb (GFC) resolution-of-the-identity (RI) DFT, pseudospectral DFT/HF, and dual-level DFT.
3. Low-scaling SCF calculation using diagonalization-free approaches: purification density matrix, pseudodiagonalization, and quadratic convergence SCF.
4. Excited-state DFT calculation: time-dependent DFT (TDDFT) and transition potential (DFT-TP).
5. Accurate electron correlation methods for ground and excited states: Møller–Plesset perturbation theory, coupled-cluster (CC) theory, and quantum Monte Carlo (QMC) method.
6. Massively parallel computing on the K computer and Intel-based architectures: HF, DFT, resolution-ofthe-identity second-order Møller–Plesset (RI-MP2) method, and QMC method.
7. Two-component relativistic electronic structure calculation with spin-orbit (SO) interactions: Douglas–Kroll (DK1, DK2 and DK3), zeroth- and infinite-order regular approximations (ZORA and IORA), and Relativistic scheme for Eliminating Small Components (RESC).
8. Model calculations for large molecular systems: quantum mechanics/molecular mechanics (QM/MM) and Our own N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM).
9. Calculation of solvation effects: COnductor-like Screening MOdel (COSMO) (interfaced with the HONDO program), averaged solvent electrostatic potential/molecular dynamics (ASEP/MD), and QM/MM-MD.
10. Efficient calculation for chemical reaction pathway.
11. Ab initio molecular dynamics calculation.
12. Calculation of magnetic properties: nuclear magnetic resonance (NMR) chemical shifts, magnetizabilities, and electron paramagnetic resonance (EPR) g tensors.
13. Population analysis: Mulliken and natural bond orbital (NBO) analysis (interfaced with NBO 6.0).
14. Orbital interaction analysis: maximally interacting orbital (MIO) and paired interacting orbital (PIO) methods.