Li Xinzheng's research group and collaborators have made new progress in the development of simulation methods for molecular cluster tunneling cleavage spectra

Professor Li Xinzheng's research group, from the Institute of Condensed Matter Physics and Materials Physics, the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, the Nanophotoelectronics Frontier Science Center, and the Light Element Advanced Materials Research Center, in cooperation with the team of Academician Zhang Donghui and Professor Jiang Ling of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and Professor Fang Wei of the Department of Chemistry of Fudan University, summarized a new method for calculating the tunneling cleavage spectrum of molecular clusters and applied it to water trimers.
Obtain accurate quantitative simulation results
that can be consistent with experimental observations with wavenumber accuracy.
On November 22, 2022, the relevant research results were officially published online in the Journal of the American Chemical Society (J.
Smith).
Am.
Chem.
Soc.
144，21356，2022）
。

Tunneling is a basic quantum effect, and the well-known Scanning Tunneling Microscope (STM) uses the sensitive dependence of electron tunneling on its barrier to detect solid surface structures
.
Corresponding to electron tunneling, atomic nuclei also tunnel due to their own quantum effects
.

In the case of ammonia, the most common umbrella molecule, the quantum effect inherent in the nucleus actually allows it to tunnel between two symmetric states and then superimpose (Figure 1a).

Correspondingly, the molecular energy level also cleaves (Figure 1b).

In fact, as early as 1929, when the basic principles of quantum mechanics were just established, R.
G.
Dickinson et al.
observed dozens of wavenumber cleavage signals in the spectrum of gaseous ammonia (Phys.
Rev.
34，582，1929）
。 In 1932, D.
M.
Dennison and G.
Dennison E.
Uhlenbeck collaborated to calculate the structural information of the ammonia molecule using the simple Wentzel–Kramers–Brillouin (WKB) method: the N-H bond length was about 1.
02 angstroms, and the H-H distance was about 1.
64 angstroms (Phys.
Rev.
41, 313, 1932).

At a time when quantum chemistry was just getting started and experimental techniques were limited, this was an astonishing accuracy (compared to the modern results of 1.
012 angstroms and 1.
624 angstroms).

The physics behind this phenomenon is so profound that in 1972 when P.
W.
Anderson also discusses symmetry at length in his famous essay "More is different" (Science 177, 393, 1972).

Now, such a concept has been widely accepted and put into basic courses such as "quantum mechanics" and "physical chemistry
" as a basic knowledge 。 In cutting-edge research, the tunneling spectroscopy measurement of molecular clusters has also achieved great success (for example, the new laser and synchrotron spectroscopy techniques, see Science 257, 1937, 1992; Science 271, 59, 1996; Science 271, 62, 1996; Science 351, 1310, 2016, etc.
).

In sharp contrast, however, precise computational methods are scarce in computational simulations due to the high-dimensional nature of this quantum process
.

Fig.
1 Schematic diagram of tunneling cleavage in a simple bipotential well corresponding to ammonia molecule flipping

In response to this problem, Li Xinzheng's research group proposed an improved path integration method to realize the full-dimensional quantum simulation
of tunneling cleavage spectra in molecular clusters.
On this basis, combined with the high-fitting precision water molecular potential energy surface of the CCSD (T) level (a high-precision quantum chemical method) developed by Zhang Donghui's team, and cooperating with Jiang Ling and Fang Wei, they obtained accurate quantitative simulation results
that can be consistent with experimental observations in wavenumber accuracy on the description of the twisting tunneling process of water trimer clusters.
This method rigorously includes the non-nearest neighbor coupling between nuclear quantum states (the coupling coefficients described by h 2 and h 3 in Figure 2) that are often ignored by the Hückel Model, and is no longer limited to the simple dynamic process of crossing a potential well tunnel from one potential well to another.
Instead, it reflects the strong non-local characteristics
of the nuclear wave packets in the system.
Correspondingly, the calculation accuracy has also been greatly improved
compared with the previous theoretical calculation.
In this work, the quality of the three-body interaction term of the new potential energy surface was also verified
first.
Since the computational amount of the path integration molecular dynamics method itself increases almost linearly with the size of the system, this method can also be directly applied to
the tunneling and cleavage problem of other systems.
If it is applied to the calculation of tetrameric pentamers of water (high-order terms such as the four-body term corresponding to the potential energy surface), it is also expected to open the way
to systematically improve the potential energy surface of water molecules with high-precision spectra as calibrators.

Fig.
2 The calculation results of twisting tunneling cleavage of water trimers are consistent
with the experimental values in the order of wave magnitude.
The subplot represents the coupling of near neighbors (_H1) and non-neighbors (_H2, H3).

Zhu Yucheng, a 2018 doctoral student at the School of Physics of Peking University, and Yang Shuo, a postdoctoral fellow at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, are the co-first authors of the paper, and Zhang Donghui and Li Xinzheng are the co-corresponding authors
of the paper.
Zeng Jiaxi, Jiang Ling and Fang Wei, 2019 doctoral students of Peking University, have made important substantive contributions
.

The research work has been supported
by the National Natural Science Foundation of China, the National Key Research and Development Program, the Beijing Municipal Natural Science Foundation, and the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences (Category B).