

THEORETICAL NANOPHYSICS Group 




JengDa Chai
(蔡政達)
Professor
Department of Physics
National Taiwan University
No. 1, Sec. 4, Roosevelt Road
Taipei 10617, Taiwan
Office: R534, New Physics Building, NTU
Phone: +886233665586
Fax: +886223639984
Email: jdchai@phys.ntu.edu.tw
Website: http://web.phys.ntu.edu.tw/jdchai/

Academic Websites:
Professional Experience:
 Professor, Department of Physics, National Taiwan University (2017.8present)
 Associate Professor, Department of Physics, National Taiwan University (2013.82017.7)
 Center Scientist, Physics Division, National Center for Theoretical Sciences (North) (2014.12014.12)
 Assistant Professor, Department of Physics, National Taiwan University (2009.82013.7)
 Postdoctoral Fellow, Department of Chemistry, University of California, Berkeley and Chemical Sciences Division, Lawrence Berkeley National Laboratory (2006.12009.6)
Education:
 Ph.D. in Chemical Physics, University of Maryland, College Park (2002.72005.12)
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 M.S. in Physics (Ph.D. Candidate), The Ohio State University (1999.92002.6)
 B.S. in Physics (with a minor in Mathematics), National Taiwan University (1993.91997.6)
Awards and Honors:
 Project for Excellent Junior Research Investigators, Ministry of Science and Technology, Taiwan (20182021)
 Excellence in Teaching Award, National Taiwan University, Taiwan (2018)
 Junior Research Investigators Award, Academia Sinica, Taiwan (2017)
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 Outstanding Young Physicist Award, The Physical Society of the Republic of China (Taiwan) (2016)
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 Project for Excellent Junior Research Investigators, Ministry of Science and Technology, Taiwan (20152018)
 Career Development Award, National Taiwan University, Taiwan (20152016)
 Youth Medal, China Youth Corps, Taiwan (2015)
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 TWAS Young Affiliate, The World Academy of Sciences (TWAS)  for the advancement of science in developing countries (20132017)
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 Career Development Award, National Taiwan University, Taiwan (20132015)
 Young Theorist Award, National Center for Theoretical Sciences, Taiwan (2012)
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 EPSON Scholarship Award, The International Society for Theoretical Chemical Physics (2011)
Software Development:
 Developer, QChem Inc. (2008present)
[Theoretical methods developed in our group may be available in QChem ]
Journal Editorial Boards:
 Editorial Board, International Journal of Quantum Chemistry (2018.3present)
 Editorial Board, Chinese Journal of Physics (2017.12present)
 Editorial Board, London Journals Press (2016.9present)
 Editorial Board, International Journal of Advanced Research in Physical Science (2016.8present)
 Editorial Board, The Open Access Journal of Science and Technology (2016.3present)
 Editorial Board, Mediterranean Journal of Physics (2016.1present)
 Editorial Board, Journal of Lasers, Optics & Photonics (2015.9present)
 Editorial Board, Open Journal of Physical Chemistry (2011.3present)
Journal Referees:
 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
 Nanoscale
 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
 Molecules
 Synthetic Metals
 Journal of Electronic Materials
 Entropy
 Chinese Journal of Physics
 Acta PhysicoChimica Sinica
 Journal of Theoretical and Computational Chemistry
 Computation
Research Interests:
"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 quantummechanical 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. KohnSham Density Functional Theory (KSDFT)
A. Goal:
a. Exchange Energy Density Functional
b. Correlation Energy Density Functional
c. LinearScaling Methods
B. Direction:
a. SelfInteraction 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. ThermallyAssistedOccupation Density Functional Theory (TAODFT)
A. Density Functional Approximations
B. Fictitious Temperature
C. Fundamental Properties
D. Extensions
E. Strongly Correlated Electron Systems at the Nanoscale
Description:
For systems with strong static correlation effects (i.e., systems possessing radical character or multireference systems), KSDFT employing the LDA, GGA, MGGA,
hybrid, and doublehybrid exchangecorrelation energy functionals may provide unreliable results, due to the inappropriate treatment of static correlation.
To accurately predict the properties of these systems, highlevel ab initio multireference electronic structure methods are typically needed. However,
accurate multireference 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 groundstate properties of strongly correlated electron systems at the nanoscale with minimum computational complexity, we have recently
developed TAODFT [1].
Unlike finitetemperature density functional theory, TAODFT is developed for groundstate systems at zero (physical) temperature (just like KSDFT).
In contrast to KSDFT, TAODFT is a density functional theory with fractional orbital occupations produced by the FermiDirac 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 TAODFT is always nonpositive, yielding insignificant contributions for a singlereference system
(i.e., a system possessing nonradical character), and significantly lowering the total energy of a multireference system. TAODFT has similar computational
cost as KSDFT for singlepoint energy and analytical nuclear gradient calculations, and reduces to KSDFT in the absence of strong static correlation effects.
Besides, existing exchangecorrelation energy functionals and dispersion correction schemes in KSDFT may also be adopted in TAODFT [2].
Recently, we have defined the exact exchange in TAODFT, and developed the corresponding global and rangeseparated hybrid schemes in TAODFT for an improved
description of nonlocal exchange effects [3]. With some simple modifications, global hybrid functionals [3] and rangeseparated hybrid functionals [3, 5] in
KSDFT can be combined seamlessly with TAODFT. Relative to TAODFAs (i.e., TAODFT with the density functional approximations), global hybrid functionals in
TAODFT 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 selfconsistent scheme for the determination of the fictitious temperature in TAODFT [4]. Relative to the systemindependent
fictitious temperature scheme in TAODFT, the selfconsistent fictitious temperature scheme in TAODFT is generally superior in performance for a very broad
range of applications.
To demonstrate the applicability of TAODFT, we have recently employed TAODFT to study the groundstate properties of various strongly correlated electron
systems at the nanoscale.
First, we have studied the electronic properties of zigzag graphene nanoribbons (ZGNRs) using TAODFT [6]. 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 nonKekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using
TAODFT [7]. Our results have revealed that the studies of Kekulé and nonKekulé 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 [9] and Möbius cyclacenes [13] using TAODFT. 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 groundstate 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 Liadsorbed acenes [8], Literminated linear carbon chains [10], and Literminated linear boron chains [11] can be
highcapacity hydrogen storage materials (HSMs) for reversible hydrogen uptake and release at ambient (or nearambient) conditions using dispersioncorrected
TAODFT. Accordingly, the search for ideal HSMs can be readily extended to large systems with strong static correlation effects.
Very recently, we have employed TAODFT to study the electronic properties of the coronene series with n fused benzene rings at each side (designated as
ncoronenes) [12]. With increasing n, there is a transition from the nonradical character of the smaller ncoronenes to the increasing polyradical character
of the larger ncoronenes. 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.
References:
a. TAODFT:
[1]
J.D. Chai*, J. Chem. Phys. 136, 154104 (2012).
b. Extensions of TAODFT:
[2]
J.D. Chai*, J. Chem. Phys. 140, 18A521 (2014).
[3]
J.D. Chai*, J. Chem. Phys. 146, 044102 (2017).
[4]
C.Y. Lin, K. Hui, J.H. Chung, and J.D. Chai*, RSC Adv. 7, 50496 (2017).
[5]
F. Xuan, J.D. Chai*, and H. Su*, ACS Omega 4, 7675 (2019).
c. Applications of TAODFT:
[6]
C.S. Wu and J.D. Chai*, J. Chem. Theory Comput. 11, 2003 (2015).
[7]
C.N. Yeh and J.D. Chai*, Sci. Rep. 6, 30562 (2016).
[8]
S. Seenithurai and J.D. Chai*, Sci. Rep. 6, 33081 (2016).
[9]
C.S. Wu, P.Y. Lee, and J.D. Chai*, Sci. Rep. 6, 37249 (2016).
[10]
S. Seenithurai and J.D. Chai*, Sci. Rep. 7, 4966 (2017).
[11]
S. Seenithurai and J.D. Chai*, Sci. Rep. 8, 13538 (2018).
[12]
C.N. Yeh, C. Wu, H. Su*, and J.D. Chai*, RSC Adv. 8, 34350 (2018).
[13]
J.H. Chung and J.D. Chai*, Sci. Rep. 9, 2907 (2019).
[Availability:
QChem 4.3 or higher
]
[Presentation:
Webinar on TAODFT
]
3. OrbitalFree Density Functional Theory
A. Goal:
a. Kinetic Energy Density Functional
b. Pseudopotentials
B. Direction:
a. Linear Response Theory
b. KEDFs in Certain Situations
c. Transferable Pseudopotentials
C. Suitable Systems:
Systems with 1,000~1,000,000 electrons (KohnSham density functional theory is inappropriate due to its high computational cost)
4. TimeDependent Density Functional Theory
A. ExchangeCorrelation Action Functional
B. Excited States
C. RealTime Electron Dynamics
D. Quantum Transport
E. Quantum Hydrodynamics
5. Materials for New Energy
A. Organic Solar Cells
B. Hydrogen Storage Materials



