THEORETICAL NANOPHYSICS Group
Department of Physics
National Taiwan University
No. 1, Sec. 4, Roosevelt Road
Taipei 10617, Taiwan
Office: R534, New Physics Building, NTU
- Professor, Department of Physics, National Taiwan University (2017.8-present)
- Associate Professor, Department of Physics, National Taiwan University (2013.8-2017.7)
- Center Scientist, Physics Division, National Center for Theoretical Sciences (North) (2014.1-2014.12)
- Assistant Professor, Department of Physics, National Taiwan University (2009.8-2013.7)
- Postdoctoral Fellow, Department of Chemistry, University of California, Berkeley and Chemical Sciences Division, Lawrence Berkeley National Laboratory (2006.1-2009.6)
- Ph.D. in Chemical Physics, University of Maryland, College Park (2002.7-2005.12)
- M.S. in Physics (Ph.D. Candidate), The Ohio State University (1999.9-2002.6)
- B.S. in Physics (with a minor in Mathematics), National Taiwan University (1993.9-1997.6)
Awards and Honors:
- Project for Excellent Junior Research Investigators, Ministry of Science and Technology, Taiwan (2018-2021)
- Excellence in Teaching Award, National Taiwan University, Taiwan (2018)
- Junior Research Investigators Award, Academia Sinica, Taiwan (2017)
- Outstanding Young Physicist Award, The Physical Society of the Republic of China (Taiwan) (2016)
- Project for Excellent Junior Research Investigators, Ministry of Science and Technology, Taiwan (2015-2018)
- Career Development Award, National Taiwan University, Taiwan (2015-2016)
- Youth Medal, China Youth Corps, Taiwan (2015)
- TWAS Young Affiliate, The World Academy of Sciences (TWAS) - for the advancement of science in developing countries (2013-2017)
E2 (p. 8),
- Career Development Award, National Taiwan University, Taiwan (2013-2015)
- Young Theorist Award, National Center for Theoretical Sciences, Taiwan (2012)
- EPSON Scholarship Award, The International Society for Theoretical Chemical Physics (2011)
- Developer, Q-Chem Inc. (2008-present)
[Theoretical methods developed in our group may be available in Q-Chem ]
Journal Editorial Boards:
- Editorial Board, International Journal of Quantum Chemistry (2018.3-present)
- Editorial Board, Chinese Journal of Physics (2017.12-present)
- Editorial Board, London Journals Press (2016.9-present)
- Editorial Board, International Journal of Advanced Research in Physical Science (2016.8-present)
- Editorial Board, The Open Access Journal of Science and Technology (2016.3-present)
- Editorial Board, Mediterranean Journal of Physics (2016.1-present)
- Editorial Board, Journal of Lasers, Optics & Photonics (2015.9-present)
- Editorial Board, Open Journal of Physical Chemistry (2011.3-present)
- Nature Communications
- Scientific Reports
- Journal of Chemical Physics
- Journal of Chemical Theory and Computation
- Physical Chemistry Chemical Physics
- Journal of Physical Chemistry Letters
- Journal of Physical Chemistry
- Chemical Science
- Journal of Materials Chemistry C
- RSC Advances
- Journal of Computational Chemistry
- International Journal of Quantum Chemistry
- Theoretical Chemistry Accounts
- Molecular Physics
- Chemical Physics Letters
- Journal of the Taiwan Institute of Chemical Engineers
- Bulletin of the Chemical Society of Japan
- Synthetic Metals
- Journal of Electronic Materials
- Chinese Journal of Physics
- Acta Physico-Chimica Sinica
- Journal of Theoretical and Computational Chemistry
"The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry
are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too
complicated to be soluble. It therefore becomes desirable that approximate practical methods of applying quantum mechanics
should be developed, which can lead to an explanation of the main features of complex atomic systems without too much
computation." P. A. M. Dirac (1929)
To meet the challenge, our group has focused on the development of new quantum-mechanical methods suitable for the study
of nanoscale systems (with 100~1,000,000 electrons), and their applications to materials for new energy (e.g., solar cells,
hydrogen storage materials). Specific research topics are the following.
1. Kohn-Sham Density Functional Theory (KS-DFT)
a. Exchange Energy Density Functional
b. Correlation Energy Density Functional
c. Linear-Scaling Methods
a. Self-Interaction Error
b. Noncovalent Interaction Error
c. Static Correlation Error
C. Suitable Systems:
Systems with 100~1,000 electrons (the Schrödinger equation and highly accurate ab initio methods are inappropriate due to their expensive computational costs)
2. Thermally-Assisted-Occupation Density Functional Theory (TAO-DFT)
A. Density Functional Approximations
B. Fictitious Temperature
C. Fundamental Properties
E. Strongly Correlated Electron Systems at the Nanoscale
For systems with strong static correlation effects (i.e., systems possessing radical character or multi-reference systems), KS-DFT employing the LDA, GGA, MGGA,
hybrid, and double-hybrid exchange-correlation energy functionals may provide unreliable results, due to the inappropriate treatment of static correlation.
To accurately predict the properties of these systems, high-level ab initio multi-reference electronic structure methods are typically needed. However,
accurate multi-reference calculations are prohibitively expensive for large systems (especially for geometry optimization). Consequently, it remains very
challenging to investigate the properties of strongly correlated electron systems at the nanoscale using conventional electronic structure methods.
Aiming to study the ground-state properties of strongly correlated electron systems at the nanoscale with minimum computational complexity, we have recently
developed TAO-DFT .
Unlike finite-temperature density functional theory, TAO-DFT is developed for ground-state systems at zero (physical) temperature (just like KS-DFT).
In contrast to KS-DFT, TAO-DFT is a density functional theory with fractional orbital occupations produced by the Fermi-Dirac distribution (controlled by
a fictitious (reference) temperature), wherein strong static correlation is shown to be explicitly described by the entropy contribution. Similar to the static
correlation energy of a system, the entropy contribution in TAO-DFT is always nonpositive, yielding insignificant contributions for a single-reference system
(i.e., a system possessing non-radical character), and significantly lowering the total energy of a multi-reference system. TAO-DFT has similar computational
cost as KS-DFT for single-point energy and analytical nuclear gradient calculations, and reduces to KS-DFT in the absence of strong static correlation effects.
Besides, existing exchange-correlation energy functionals and dispersion correction schemes in KS-DFT may also be adopted in TAO-DFT .
Recently, we have defined the exact exchange in TAO-DFT, and developed the corresponding global and range-separated hybrid schemes in TAO-DFT for an improved
description of nonlocal exchange effects . With some simple modifications, global hybrid functionals  and range-separated hybrid functionals [3, 5] in
KS-DFT can be combined seamlessly with TAO-DFT. Relative to TAO-DFAs (i.e., TAO-DFT with the density functional approximations), global hybrid functionals in
TAO-DFT are generally superior in performance for a wide range of applications, such as thermochemistry, kinetics, reaction energies, and optimized geometries.
Very recently, we have proposed a self-consistent scheme for the determination of the fictitious temperature in TAO-DFT . Relative to the system-independent
fictitious temperature scheme in TAO-DFT, the self-consistent fictitious temperature scheme in TAO-DFT is generally superior in performance for a very broad
range of applications.
To demonstrate the applicability of TAO-DFT, we have recently employed TAO-DFT to study the ground-state properties of various strongly correlated electron
systems at the nanoscale.
First, we have studied the electronic properties of zigzag graphene nanoribbons (ZGNRs) using TAO-DFT . The ground states of ZGNRs are found to be singlets
for all the widths and lengths studied. The longer ZGNRs should exhibit increasing polyradical character in their ground states, with the active orbitals
being mainly localized at the zigzag edges.
Second, we have investigated the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using
TAO-DFT . Our results have revealed that the studies of Kekulé and non-Kekulé structures qualitatively describe the radical character of alternant PAHs,
which could be useful when electronic structure calculations are infeasible due to the expensive computational cost.
Third, we have studied the electronic properties of cyclacenes  and Möbius cyclacenes  using TAO-DFT. Similar to acenes, the ground states of cyclacenes
and Möbius cyclacenes are singlets for all the cases studied. In contrast to acenes, the electronic properties of cyclacenes and Möbius cyclacenes, however,
exhibit oscillatory behavior for the smaller cyclacenes and Möbius cyclacenes in the approach to the corresponding properties of acenes with increasing number
of benzene rings. The larger cyclacenes and Möbius cyclacenes should exhibit increasing polyradical character in their ground states, with the active orbitals
being mainly localized at the peripheral carbon atoms. Interestingly, the ground-state geometry of Möbius cyclacene is composed mainly of an essentially
untwisted open chain plus a highly twisted stripe. In other words, the twist is not evenly distributed along the whole chain.
In addition, we have shown that Li-adsorbed acenes , Li-terminated linear carbon chains , and Li-terminated linear boron chains  can be
high-capacity hydrogen storage materials (HSMs) for reversible hydrogen uptake and release at ambient (or near-ambient) conditions using dispersion-corrected
TAO-DFT. Accordingly, the search for ideal HSMs can be readily extended to large systems with strong static correlation effects.
Very recently, we have employed TAO-DFT to study the electronic properties of the coronene series with n fused benzene rings at each side (designated as
n-coronenes) . With increasing n, there is a transition from the non-radical character of the smaller n-coronenes to the increasing polyradical character
of the larger n-coronenes. Moreover, the latter should be closely related to the localization of active orbitals at the zigzag edges, which increases with the
increase of the side length.
J.-D. Chai*, J. Chem. Phys. 136, 154104 (2012).
b. Extensions of TAO-DFT:
J.-D. Chai*, J. Chem. Phys. 140, 18A521 (2014).
J.-D. Chai*, J. Chem. Phys. 146, 044102 (2017).
C.-Y. Lin, K. Hui, J.-H. Chung, and J.-D. Chai*, RSC Adv. 7, 50496 (2017).
F. Xuan, J.-D. Chai*, and H. Su*, ACS Omega 4, 7675 (2019).
c. Applications of TAO-DFT:
C.-S. Wu and J.-D. Chai*, J. Chem. Theory Comput. 11, 2003 (2015).
C.-N. Yeh and J.-D. Chai*, Sci. Rep. 6, 30562 (2016).
S. Seenithurai and J.-D. Chai*, Sci. Rep. 6, 33081 (2016).
C.-S. Wu, P.-Y. Lee, and J.-D. Chai*, Sci. Rep. 6, 37249 (2016).
S. Seenithurai and J.-D. Chai*, Sci. Rep. 7, 4966 (2017).
S. Seenithurai and J.-D. Chai*, Sci. Rep. 8, 13538 (2018).
C.-N. Yeh, C. Wu, H. Su*, and J.-D. Chai*, RSC Adv. 8, 34350 (2018).
J.-H. Chung and J.-D. Chai*, Sci. Rep. 9, 2907 (2019).
Q-Chem 4.3 or higher
Webinar on TAO-DFT
3. Orbital-Free Density Functional Theory
a. Kinetic Energy Density Functional
a. Linear Response Theory
b. KEDFs in Certain Situations
c. Transferable Pseudopotentials
C. Suitable Systems:
Systems with 1,000~1,000,000 electrons (Kohn-Sham density functional theory is inappropriate due to its high computational cost)
4. Time-Dependent Density Functional Theory
A. Exchange-Correlation Action Functional
B. Excited States
C. Real-Time Electron Dynamics
D. Quantum Transport
E. Quantum Hydrodynamics
5. Materials for New Energy
A. Organic Solar Cells
B. Hydrogen Storage Materials