Truhlar Research Group News

Yesterday, Research.com informed Professor Truhlar that he has been recognized with their Chemistry Leader Award for 2023.  Research.com recently released the 2023 Edition of the Ranking of Top Scientists in the field of Chemistry:
https://research.com/scientists-rankings/chemistry

Professor Truhlar ranked 7th in the world and 4th in United States.

Today was published our invited perspective article in JCTC:
Y. Shu and D. G. Truhlar, Journal of Chemical Theory and Computation 19, 380-395 (2022).
It is openly available to all readers at doi.org/10.1021/acs.jctc.2c00988.

In this perspective, we introduced the theoretical concepts that are essential to understand decoherence in electronically nonadiabatic molecular events. We emphasized that the reduced density matrix of a subsystem evolves according to a nonunitary Liouville–von Neumann equation even if the full density matrix of the combined subsystem and environment remains pure. When governed by a nonunitary Liouville–von Neumann equation, the off-diagonal elements of the subsystem reduced density matrix decay to zero. This is the central fact of decoherence; and this decay of coherence is essential in understanding the propagation of the electronic reduced density matrix in chemical and physical systems, not just in a condensed phase due to the solvent but even in a small molecule in the gas phase, where the electronic subsystem is decohered by the nuclei, which act as an environment. We discussed how the continuous monitoring of electrons by the nuclei causes decoherence of the reduced electronic density matrix.

Simulations of the decoherence effect in nonadiabatic dynamics can be achieved by a combination of a decay-of-mixing algorithm and an ensemble average over initial conditions, for example by decoherence with decay of mixing, which is available in the ANT program and will soon become available in SHARC-3.0.

Decoherence was also recently discussed (very briefly) in Don Truhlar’s letter to the editor of Physics Today: “More on the Quantum Measurement Problem,” D. G. Truhlar, Physics Today 75(11), 13 (Nov. 2022) doi.org/10.1063/PT.3.5113

Y. Liu, C. Zhang, Z. Liu, D. G. Truhlar, Y. Wang, and X. He, Nature Computational Science 3, 48-58 (2023).

The paper is available as an open access publication at
doi.org/10.1038/s43588-022-00371-5

Kohn–Sham density functional theory (KS-DFT) has been widely used in various fields of chemistry, but no functional can accurately predict the whole range of chemical properties. A universal functional that has high across-the-board accuracy for both main-group elements and transition metals is highly desirable for a broad range of chemical applications.

In this work, we optimized an exchange-correlation functional, CF22D, with high across-the-board accuracy for chemical applications by using a flexible functional form that combines a global hybrid meta-nonseparable gradient approximation that depends on density and occupied orbitals with a damped dispersion term that depends on geometry. In the spirit of machine learning, we optimized the energy functional by using a big database and active learning. The results illustrate the power of such an approach. As compared to selected but diverse previous functionals we find excellent performance. The new functional has the best overall performance for the GMTKN55 database (consisting of 55 diverse datasets) with a mean unsigned error (MUE) of 1.45 kcal/mol, and it performs well for both radical systems and nonradical systems in the GMTKN55 database. The CF22D functional gives competitive performance to the recently developed deep-learning functional DM21 from DeepMind (Science 374, 1385–1389 (2021)). It gives excellent performance for the AME418 dataset of Minnesota Database 2019 with an MUE of 2.10 kcal/mol, and it is competitive with ωB97M-V for the MGCDB84 database, especially for noncovalent ‘difficult’ systems and thermochemistry. We combined all the data points used in databases with more transition metal data to create a large, combined database DDB22 (Diverse Database 2022). For DDB22, among the compared functionals, CF22D gives the best overall results for barrier heights, isomerization energies, thermochemistry, and noncovalent interactions and the best results for the transition metal datasets that were used only for testing. Furthermore, CF22D provides the long-range van der Waals tail of potential energy curves for noncovalent interactions, and it gives good performance for vertical excitation energies, dipole moments, and molecular structures. Overall, CF22D is superior to most widely used functionals for a broad range of databases.

CF22D can be recommended for applications involving a broad range of bonding and noncovalent interactions of both main-group compounds and transition-metal compounds, which makes it appropriate for studies of catalysis, functional materials, biochemistry, and environmental chemistry.

On December 13, Siriluk Kanchankungwankul defended her these entitled “Electronic Structure Theory and Computations: Application of Density Functional Theory to Heterogeneous Catalysis and Density Functional Development.” Siri is moving on to a position at DuPont Silicon Valley Technology & Innovation Center in Sunnyvale, CA.

On December 16, Jiaxin Ning defended her these entitled “Electronic Structures of Lanthanide Compounds and Aromatic Molecules.” Jiaxin is moving on to a position in Research & Development at Bytedance Inc. in Mountain View, CA.

This news article was originally published on the Department of Chemistry News.

Our feature article on diabatic states has now been published:

“Diabatic States of Molecules,” Y. Shu, Z. Varga, S. Kanchanakungwankul, L. Zhang, and D. G. Truhlar, Journal of Physical Chemistry A 126, 992-1018 (2022). doi.org/10.1021/acs.jpca.1c10583

Quantitative simulations of electronically nonadiabatic molecular processes require both accurate dynamics algorithms and accurate electronic structure information. Very often semiclassical nonadiabatic dynamics is performed in a direct way, in which one evaluates the adiabatic potentials, gradients, and couplings from electronic structure at every time step. Direct semiclassical nonadiabatic dynamics is expensive due to the high cost of electronic structure calculations. And hence it is limited to small systems or lower levels of electronic structure methods. In addition, there is an increasing interest in involving nuclear quantum effects. The goals of performing electronically nonadiabatic quantum dynamics with quantitatively accurate electronic structure input and performing electronically nonadiabatic semiclassical dynamics with high levels of theory, long simulation times, and sufficient ensemble averaging have stimulated the development of diabatic representations. Diabatic representations, unlike adiabatic representations, are not unique. But they are very convenient because, in a diabatic representation, the population transfer between electronic states is governed by the diabatic coupling, which – unlike the coupling vector in the adiabatic representation – is a smooth scalar; therefore diabatic potential energy matrices can be fitted to smooth analytic functions. The goal of this article is to review the utility of diabatic representations for dynamics, the characteristics of a diabatic representation, the connections between diabatization methods, and the recent developments of new methods from our group.

On Oct. 9-12, 2021, the Chinese Chemical Society National Conference on Quantum Chemistry was held in Shanghai. Several group alumni who participated are shown in the accompanying picture, which shows, left to right: Xiao He, Xuefei Xu, Yan Zhao, Xin Zhang, and Xin-Ping Wu. The titles of the presentations they made were as follows:   

• Xiao He: “Towards Accurate Simulation of Complex Systems”
• Xin-Ping Wu: “Developing QM/MM Methods for Metal-Organic Frameworks”
• Xuefei Xu: “Theoretical and Computational Study of Gas-Phase Chemical Reaction Dynamics” 
• Yan Zhao: “Development of Quantum Chemical Methods and their Applications to Energy and Environmental Materials”

Our new computer program, TUMME, solves the master equation by the method of chemically significant eigenmodes to calculate temperature- and pressure-dependent rate constants for thermal unimolecular chemical reactions and chemical activation reactions. A paper describing the program is now published:

R. M. Zhang, X. Xu, and D. G. Truhlar, TUMME: Tsinghua University Minnesota Master Equation program, Computer Physics Communications 270, 108140/1-17 (2021).
doi.org/10.1016/j.cpc.2021.108140

Share link for free access (expires Oct. 30, 2021):
https://authors.elsevier.com/a/1djlI2OIngMWU

The program, which includes a detailed manual, is available free of charge at https://comp.chem.umn.edu/tumme

TUMME calculates microcanonical flux coefficients (which are inputs to the master equation) by multi-structural variational transition state theory with small-curvature tunneling (MS-VTST/SCT) or by conventional transition state theory or RRKM theory (which is conventional transition state theory applied to unimolecular reactions) by using data read from output files of Gaussian, Polyrate, and/or MSTor.

The program is written in double precision with Python 3, and quadruple and octuple precision are also available for some subtasks in C++ (the higher precision is often necessary at low temperature or for competing reactions). The  Python code can run in serial or parallel (multithreading and MPI), and the C++ code can run on a single processor or on multiple processors with OpenMP.

Weighted flux coefficients k_X,wt for unimolecular dissociation of 2-methylhexyl radicals at 700 K and 10^-6 torr; E_η is internal energy, and X equal to A, B, or C denotes one of three possible products. Double precision refers to 8 bytes per word, and quadruple precision refers to 16 bytes per word.

Dayou Zhang is the recipient of the 2021-2022 Robert and Jill DeMaster Excellence Fellowship. This is a very competitive fellowship and is a strong endorsement of his accomplishments so far in our graduate program.

Dayou’s work has already led to four publications:

1. “Spin Splitting Energy of Transition Metals: A New, More Affordable Wave Function Benchmark Method and Its Use to Test Density Functional Theory,” D. Zhang and D. G. Truhlar, Journal of Chemical Theory and Computation 16, 4416-4428 (2020). doi.org/10.1021/acs.jctc.0c00518

2. “Unmasking Static Correlation Error in Hybrid Kohn–Sham Density Functional Theory,” D. Zhang and D. G. Truhlar, Journal of Chemical Theory and Computation 16, 5432-5440 (2020). doi.org/10.1021/acs.jctc.0c00585

3. “Multiconfigurational Effects on the Density Coherence,” D. Zhang and D. G. Truhlar, Journal of Chemical Theory and Computation 16, 6915-6925 (2020). doi.org/10.1021/acs.jctc.0c00903

4. “Multiconfiguration Density-Coherence Functional Theory,” D. Zhang, M. R. Hermes, L. Gagliardi, and D. G. Truhlar, Journal of Chemical Theory and Computation 17, 2775-2782 (2021). doi.org/10.1021/acs.jctc.0c01346

Paper 1 presents a new algorithm, CASPT2.5, for accurate calculations on strongly correlated systems.  It also tests this algorithm and several other multireference methods on transition metal spin-splitting energies for which the standard CCSD(T) method fails.  The CASPT2.5 method is found to be very accurate with an affordable cost for small molecules and it is used to provide benchmark spin-splitting energies and geometries for transition metal complexes. These were used to test 60 exchange-correlation functionals. The MN15-L method was validated to perform very well for spin splitting.  We believe that CASPT2.5 and MN15-L can be very useful for calculating spin splittings, which are important for many technical applications.

Accurately treating strongly correlated systems is a challenging task for Kohn–Sham density functional theory (KS theory). The factors underlying its inaccuracy are only partly clear, especially regarding static correlation. To unmask the static correlation error, paper 2 compares the potential energy curves of four diatomic molecules, namely H₂, F₂, HF, and NaF, using restricted and unrestricted KS theory. With the aid of restricted KS theory calculations, Dayou found that limiting the percentage of Hartree–Fock exchange significantly reduces the static correlation error in Hartree–Fock theory. This work extends and builds upon a stimulating paper of Nobel Laureate Martin Karplus.  Dayou showed that utilizing restricted KS calculations is a useful tool to elucidate the origin of static correlation error in KS theory. One reviewer commented, “It is useful that it is brought home to the readers that there may be important drawbacks to the inclusion of exact exchange, and what the problems are exactly.” Another reviewer said, “I think this is a very interesting perspective on static correlation and I think the authors do a good job of supporting their claims with calculations.”

Paper 3 is a study of density coherences in multiconfiguration self-consistent field theory and Kohn−Sham density functional theory. The density coherences under study here are the off-diagonal elements of the one-body density matrix. Although the one-body density matrix is a central quantity in wave function and density functional methods, it has not been nearly as widely studied as the density itself, which is the diagonal part of the density matrix in the coordinate representation. Dayou compared CASSCF, Hartree–Fock, local Kohn–Sham theory, and hybrid Kohn–Sham theory for the density coherence.  The quantitative conclusions are quite striking, showing larger deviations among the methods than one might have expected. We expect that there will be much more emphasis on the density matrix in the near future, and this study can lay a groundwork for general expectations and thereby help guide the development work.

Paper 4 describes a new theory — multiconfiguration density coherence functional theory — and it demonstrates its promise for representing the correct physics of static and dynamic correlation. Potentially it allows new strategies for designing density matrix functionals that take advantage of the physical interpretation of the number of unpaired electrons rather than unphysical effective spin densities. Dayou’s paper shows that already with just two parameters he can get good results for bond energies, equilibrium distances, vibrational frequencies, and potential curves.

In addition to his published work, Dayou has completed a fifth project and has a sixth in progress.  Project 5 analyzes static and dynamic electron correlation by decomposing the total electronic energy of calculations by restricted Hartree–Fock (RHF) theory, complete active space self-consistent field (CASSCF) theory, and multireference configuration interaction (MRCI). Dayou is using two different schemes to break down the total energy contributions to the potential energy curves for the dissociation of diatomic molecules. He finds that a significant portion of the static correlation comes from the part of the energy that is not expressible in terms of the one-body reduced density matrix. He also finds that negligible static correlation is included in the sum of the effective one-electron energy and the classical two-electron energy, which provides a way to understand the success of multiconfiguration pair-density functional theory (MC-PDFT) and multiconfiguration density coherence functional theory (MC-DCFT). This kind of analysis of energetic contributions to the total energy in terms of different components of the two-body reduced density matrix has never been done before, and it leads to fundamental understanding.

In project 6, Dayou is developing new strategies for multi-parameter optimization of density coherence functionals.

Jiaxin Ning has been awarded the 2021 John Overend Award for Graduate Research in Physical Chemistry. Jiaxin was given this award in recognition of her outstanding work on the following two research articles:

“The Valence and Rydberg States of Dienes,” J. Ning and D. G. Truhlar, Physical Chemistry Chemical Physics 22, 6176-6183 (2020). doi.org/10.1039/c9cp06952f

 “Spin-Orbit Coupling Changes the Identity of the Hyper-Open-Shell Ground State of Ce⁺, and the Bond Dissociation Energy of CeH⁺ Proves to be Challenging for Theory,” J. Ning and D. G. Truhlar, Journal of Chemical Theory and Computation 17, 1421−1434 (2021). doi.org/10.1021/acs.jctc.0c01124

Polychromophoric assemblies are widespread in both biological systems and functional materials, and 1,3-cyclohexadiene and 1,4-cyclohexadiene are classic prototypes for investigating the electronic structures of molecules with interacting conjugated and unconjugated double bonds, respectively. The existence of Rydberg states interspersed with the valence states makes the quantum mechanical calculation of their spectra very challenging and has led to uncertainty about the extent of valence–Rydberg mixing. This paper, for the first time, demonstrates that one can calculate the whole spectrum of valence and Rydberg states in a consistent fashion; the agreement of theory with experiment is remarkable. A special characteristic of Jiaxin’s analysis is the calculation of the second moments of the excited-state orbitals.  These moments give a more accurate picture of the diffuseness of the excited-state orbitals in these prototype molecules than had previously been available.

The second paper describes calculations that open new pathways for treating bonds to heavy metals. Because Ce is in the sixth row of the periodic table, relativistic effects must be treated consistently in the molecule and the dissociated ion, and the treatment of Ce⁺ turned out to require the development of new strategies. Jiaxin found that the ground state is different with and without spin-orbit coupling. She also found that the ground doublet state of Ce⁺ is an intra-atomic hyper-open-shell state. Out of 40 multireference and single-reference methods tested, only seven get the identity of the spin-orbit-free ground state right.  The dissociation energy calculations show that the quantitative chemistry of bonds to sixth-period metal atoms presents a serious challenge to quantitative quantum chemistry.

Jiaxin is currently working on the simulation of the photodissociation of 2-fluorothiophenol. Because of the unique role of the intramolecular hydrogen bond, photodissociation and internal relaxation of this molecule provide a prototype for understanding substituent effects on the important p → σ* pathway that is key to understanding photostability in biological molecules. Jiaxin is developing a unique computational procedure for using the coherent switching with decay of mixing (CSDM) method in the SHARC program to do the dynamics simulations with the XMS-CASPT2 method for the electronic structure input.

Jiaxin came to our group after earning a bachelor's degree in chemistry from Nankai University in Tianjin, China. We look forward to her continuing contributions to theoretical and computational physical chemistry.