Research (English)

metal/water interface

As typified by the electrolysis of water and the fuel-cell reaction, many interesting phenomena occur at the electrode/solution interface. Comprehensive understanding, however, has been the target of study since mid 19-th century, Recent first-principles simulation is able to follow the chemical dynamics in detail. The first simulation has been applied to the Volmer step of the hydrogen evolution reaction, revealing important role played by the hydrogen bond network of water formed on the Pt surface. For detail please click.

  • O. Sugino, I. Hamada, M. Otani, Y. Okamoto, and T. Ikeshoji, “First-Principles Molecular Dynamics Simulation of Biased Electrode/Solution Interface”, Surf. Sci. 601, 5237 (2007)
  • M. Otani, I. Hamada, O. Sugino, Y. Okamoto, and T. Ikeshoji, “Electrode Dynamics from First Principles”, J. Phys. Soc. Jpn. 77, 0248021 (2008).

Effective Screening Medium(ESM)

The above mentioned simulation was done by developing a novel multi-scale computational scheme called “effective screening medium (ESM)”, where the bulk water is modeled by a continuum of dielectric constant. This approach allows to manipulate the biased and charged-up interfaces, whereby enabling to simulate the electric double layer (Helmholtz layer) with reasonable reality.

The ESM scheme has proven effective in simulating the metal/water interface, but also made simpler the charged-up surfaces in general.

  • M. Otani and O. Sugino, “First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approach”, Phys. Rev. B 73 (11) 115407 (2006)

ESM Homepage is now available. SIESTA module (SIESTA+ESM) souce code can be downloaded.

Excitation Energy by TDDFT-Modified Linear-response

The time-dependent densityu functional theoery (TDDFT) has attracted attention as a new scheme to compute the excited states. The scheme needs to assume how the correlation and exchange of the electrons in advance, but present knowledge on the dependence, or the functional form, is limited and one usually use the so-called adiabatic local density approximation (ALDA). In spite of this, ALDA is know to work reasonably well for low-lying valence excitations but is deteriorated significantly when charge-transfer or Rydberg excitations are concerned.

As a practical scheme to overcome, we have proposed “modified linear response scheme”. The scheme has been tested to various small molecules with reasonable success.

  • Chunping Hu, Osamu Sugino, and Yoshiyuki Miyamoto, “Modified Linear Response for Time-Dependent Density Functional Theory: Application to Rydberg and Charge-Transfer Excitations”,Phys. Rev. A 74 032508 (2006).
  • Chunping Hu, Osamu Sugino, “Average excitation energies from time-dependent density functional response theory,” J. Chem. Phys. 126, 074112 (2007).

Nonadiabatic Couplings from TDDFT Linear-response

When studying the excited state dynamics, transition (mixing) between the adiabatic states is often the important issue. This problem, known as the ‘Landau-Zener’, has long been studied in many contexts, but the transition (mixing) probabilities are generally difficult to compute.

Roughly speaking, the transition probability is proportional to the velocity of the moving atoms and the non-adiabatic copupling (NAC), which specifes rate of the mixing. NAC is usually defined by the nuclear derivative operator applied to the many-body wavefunctrion, but can be alternatively defined in terms of a dynamic response of the density matrix. The response can be calculated within the density functional theory (DFT). When formulated in this way the computational difficulties are greatly reduced.

The above idea was first given by Mukamel and was first applied to the hydrogen trimer by Baer. When combined the idea with the Casida formalism, on the other hand, the computational cost and the numerical accuracy are improved. We have shown that with the combined scheme the scheme can be practically applied to larger molecules with success.

  • Chunping Hu, Hirotoshi Hirai, and Osamu Sugino, “Nonadiabatic couplings from time-dependent density functional theory: Formulation in the Casida formalism and practical scheme within modified linear response”, J. Chem. Phys. 127 064103 (2007).
  • Chunping Hu, Hirotoshi Hirai, and Osamu Sugino, “Nonadiabatic couplings from time-dependent density functional theory: II. Successes and challenges of the pseudopotential approximation”, J. Chem. Phys. (2008).

Predicting the phase transition (multicanonical method)

Materials have the crystalline, liquid, and gas phases, and predicting the phase transitions is an important issue of the first-principles calculation. By comparing the free-energy, one can predict the phase boundary. Computing the free-energy is, however, a difficult problem because the entropy needs to be evaluated equally well for all the phases. Recently we have adopted the multicanonical ensemble method to improve the existing computational scheme.

  • Yoshihide Yoshimoto, “Extended multicanonical method combined with thermodynamically optimized potential: Application to the liquid-crystal transition of silicon”, J. Chem. Phys. 125, 184103 (2006).

See also

  • O. Sugino and R. Car, “Ab initio molecular dynamics study of first-order phase transitions: melting of silicon” Phys. Rev. Lett. 74, 1823 (1995).