HPCI magazine vol.6

First-Principles
Molecular Dynamics
Simulations for WaterInterface Science
When water comes in direct contact with another substance, the “water interface” describes thearea where their respective molecules meet. It is a place rich with peculiar chemical interactions
whose underlying mechanisms are not well understood. Tatsuhiko Ohto at Osaka University isexploring the mysteries of the water interface. He uses first principles molecular dynamics(FPMD) simulations based on "density-functional theory”. These FPMD simulations computethe interactions of individual molecules based on their electronic states. Using a combinationof massive simulations on high performance computers (HPC)*1 and verification testing, his
research is expanding the frontiers of FPMD, elucidating photo-catalytic reactions and finding
the computational methods most suitable for simulating the water interface.

Tatsuhiko Ohto

Assistant Professor
Division of Frontier Materials Science
Department of Materials Engineering Science
Graduate School of Engineering Science, Osaka University*1 High Performance Computing uses
supercomputers to perform massive-scalecalculations, typically to simulate complex
real-world interactions for research anddevelopment work.

The Water Interface:
Where Chemical Interactions
Get Interesting
The “ water interface” is the molecular boundary
between water and the other substances thatwater comes in contact with. Some of thephenomena that occur here are unique to thewater interface, and not seen in either substance by itself. The surface tension between
the legs of an insect that walks on waterand the water surface, and the ability of lotus
flower leaves to repel water, are examples ofwater interface phenomena.
The interface between water and solids isalso where the electrochemical and catalytic
reactions needed for industrial technologies

01take place. The chemical properties where
an electrode meets the electrolyte in abattery, for example, can significantly
impact battery performance. And understanding the interface between catalysts andwater molecules is important for generating
clean hydrogen energy more efficiently.
Tatsuhiko Ohto at Osaka University isusing first principles molecular dynamics
simulations to elucidate the chemicalinteractions at the water interface.
“The interface region is no more than a
few water molecules thick. That very
narrow region is extremely difficult toobserve through experiments. But computer simulations allow us to reproduce
those interactions down to the movementof molecules and even individual atoms.
These simulations are proving to be an
extremely powerful tool for studying whatoccurs at the interface.” says Ohto.

Simulations Track theMovement of IndividualMolecules
Most molecular dynamics simulations todate have approximated these atomic
interactions by relying on simple functions,such as that of a spring-like force. These
are approximations of classical force fields.
However, these spring constants and otherknown parameters might only apply towater molecules away from the interface.
There was no guarantee that these values
would also hold true for the peculiar place
where water interfaces with anothermaterial.
Ohto therefore ran “first principles
molecular dynamics” (FPMD) simulations
based on deeper fundamentals,
specifically the force values obtained
through “density-functional theory”
quantum mechanical modelling of themolecules’ electronic structures, rather
than parameters obtained throughexperiments.
“Even if the simulations tell us what themolecular structure of an interface shouldbe, it’s still just a theory until it’s proventhrough real world observations. So,
we ran experiments using ‘sum frequency

vol.6 - First-Principles Molecular Dynamics Simulations for Water Interface Scienceg e n e r a t i o n ’ s p e c t r o s c o p y, w h i c hmeasures how the molecules vibrate att h e i n t e r f a c e . We t h e n c o n f i r m e d t h einterface properties predicted by ourcomputations by comparing them to theexperimental results.”

02

Explanation 1

Sum Frequency Generation Spectroscopy
Experiments

Ohto used “sum frequency generation” spectroscopy to verify the results of his FPMD
simulations. In sum frequency generation, visible light and infrared light are directed onto
the same water interface sample at the same time. The light reflected back from thatinterface is measured. When the frequency of the infrared light matches the vibration
frequency of molecules at the interface, the interface generates light at a third frequency,
which is the sum of the original two frequencies, hence “sum frequency generation”. The
frequency of the infrared light is varied to obtain the shape and intensity of the reflected light
spectrum. He then compared those results with the computer simulation findings.

Sum frequency
generation signalIR light
Visible light

SamplePhotograph (left) and diagram (right) of the experimental setup for the sum frequency generation spectroscopy measurements. The measurements were conducted by Ohto’s collaborators
at the Max Planck Institute for Polymer Research in Germany.

Experiment results
Sum frequency
generation intensity

Explaining the Mysteries of
Photo-catalyst Interfaces
Titanium dioxide is a commonly used photo-catalytic material.
Under normal conditions, titanium dioxide is “hydrophobic”,
meaning it repels water. But when exposed to ultraviolet light, it
becomes “hydrophilic”, attracting water. This change was thought
to be caused by “OH groups” (bonded pairs of oxygen andhydrogen atoms) dangling on the surface of the titanium dioxide
at the interface. But it had yet to be confirmed.
This prompted Ohto to run FPMD computations simulatingtitanium dioxide with and without OH groups dangling on its
surface, and compare those results with the actual sum
frequency generation spectra obtained through experiments. Theresults showed that the hydrophilic or hydrophobic properties
correlate strongly to the presence or absence of OH groups. This
gives us a better understanding of the photo-catalytic reactions
occurring at this interface, and may allow us to produce more
effective photo-catalysts in the future.

Free OH groups

Hydrogen-bondedOH groups
Wavenumber (cm-1 )Sum frequency
generation intensity

Simulation results(revPBE-D3(0))

Wavenumber (cm-1 )Comparison of the sum frequency generation spectra of the water-air interface from themeasurements (above) and the FPMD simulations (below). Both show peaks at frequencies
around 3500 and 3700 cm -1. These frequencies are thought to correspond to the stretchingvibrations of the hydrogen-oxygen bonds in the water molecules at the interface (positive andnegative peaks representing the upward and downward orientations of the OH bonds).

vol.6 - First-Principles Molecular Dynamics Simulations for Water Interface Science03

Explanation 2First-Principles MD Simulations of theTitanium Dioxide/Water Interface
Ohto performed FPMD simulations with and without
OH groups dangling on the surface of the titanium
dioxide (see figure below). The measured spectra
more closely matched the computer simulations withOH groups present (figure c) than without (figure a).
This suggests that OH groups are present on thesurface of UV-irradiated titanium dioxide.

Sum frequency
generation intensity

Sum frequency
generation intensity

IR frequency (cm-1 )Results of FPMD simulationsof the titanium dioxide/water interface.
(a) Depth-dependent sum frequency generation
spectra of the OH bonds calculated with FPMD
trajectories at the nonhydroxylated titanium
dioxide/water interface. (b) A snapshot of thenonhydroxylated titanium dioxide/water interface.
(c) Individual contributions of the O–H hydrogenbonded to O b atom and the O b –H groupchemisorbed on the five coordinated Ti atom tothe sum frequency generation spectrum at thehydroxylated titanium dioxide/water interface.
(d) A snapshot of the hydroxylated titanium
dioxide/water interface.
S. Hosseinpour, T. Ohto et al., J. Phys. Chem. Lett. 8, 2195 (2017).

vol.6 - First-Principles Molecular Dynamics Simulations for Water Interface Science

The University of Tokyo’s massively parallel
Oakbridge-CX supercomputer and theOsaka University Cybermedia Center ’s
OCTOPUS supercomputer were usedfor these FPMD simulations. “HPC isessential for the enormous and complex
computations that FPMD requires,” says
Ohto.

Finding Cost-Effective
Calculation Methods
Because it is not possible to get an exact
solution for a “many-body problem”, firstprinciples calculations require someamount of approximation. “Densityfunctional theory” is used for this purpose.
While the exact expression of the“exchange-correlation functional” that
describes the interaction between theelectrons of interest is not known, various
approximations have been proposed.
In general, the more accurate theapproximation, the higher its computationalcost. So, cost performance is of particularimportance for FPMD simulations, which
can require tens of thousands of calculation
steps. However, it was not well known whichapproximation formula would provide themost cost-effective calculations for such a
distinctive subject of study as materialinterfaces.

Ohto ran water interface simulations tocompare the accuracy and computation
time of different approximation formulas. He
found that some approximation methods
with relatively small computational cost canprovide highly accurate results. “Movingforward, we intend to apply these findings
to our water interface research,” says Ohto.

Explanation 3

Comparing Approximation Methods
for Exchange-Correlation Functionals(a) Accuracy in approximating
the water-air interface(b) Computational cost

Comparing various approximations of the exchangecorrelation functional for FPMD computations of thewater interface. Figure (a) on the left shows the accuracyof the calculations (the longer the radius, the more accurate).
Figure (b) on the right shows the computational cost (thelonger the radius, the longer the calculation time). For
example, “revPBE” was found to provide highly accurate
approximations with relatively short computation times.
T. Ohto et al., J. Phys. Chem. Lett. 10, 4914 (2019).

04

Toward More Realistic
Water Interface Simulations
The findings of this research provided
insights regarding the special properties of
the water interface, and also showed theenormous potential of high performancecomputing for first principles molecular
dynamics simulations. Ohto says he hopes
to further develop these methods for more
complex and more realistic studies of thephysical phenomena that occur at theinterface.
“We wer e able to find g o o d ca lcu lat io nmethods for the interface between waterand air this time,” said Ohto. “In the future,
I’d like to apply these methods to more
complex water interfaces. In real world
chemical reactions, the pH level varies
according to the ions, and there are
numerous impurities and defects on thesurface of a solid. I’ d like to run simulationsof water interfaces that are even closer toreality and take those effects into account.”

About theResearcher
Tatsuhiko Ohto says he’s been interested in thenatural sciences since childhood. He became
curious about astronomy in elementary school,and environmental issues in junior high and highschool. An internship at Germany’s world-leadingchemical manufacturer BASF during his master’s
program led to his interest in research work.
Having experienced R&D work in BASF ’s
computing department, he says he felt that,“there was still a lot left to do in academia”.
Upon returning to his university laboratory, andwanting to do research on bigger issues that
could be verified experimentally, he decided topursue his long-standing interest in catalytic
reactions. “There aren’t a lot of people working on first principles molecular dynamics,
but Fugaku and the rest of Japan’s highperformance computing infrastructure havehelped make it available to more people.
I think it ’s fascinating to be able to trace
the movement of molecules and atoms. I hopemore people will join in this field.”

Associated Research Projects• “Development of an Exchange-correlation Functional to Describe Solid-Liquid Interfaces” (hp200081)

vol.6 - First-Principles Molecular Dynamics Simulations for Water Interface Science05HPCI magazine FUGAKU HYAKKEI vol.6

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