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Electronic structure of methyl radical photodissociation

  • Aiswarya M. Parameswaran , Dayou Zhang and Donald G. Truhlar ORCID logo EMAIL logo
Published/Copyright: August 27, 2025
Pure and Applied Chemistry
From the journal Pure and Applied Chemistry Volume 97 Issue 10

Abstract

The calculation of accurate excitation energies and potential energy surfaces of photochemical reactions is a major challenge of current quantum chemistry, especially when one wants to keep the cost low enough to make detailed dynamical simulations affordable. Methods that might be affordable for large complex molecules can be tested against benchmark results for smaller molecules, where accurate benchmarks can be available. Here we report such testing for the excitation and dissociation of the methyl radical, yielding both singlet and triplet CH2. The emphasis is on multistate pair density functional theory using compressed-multistate (CMS-PDFT) and linearized (L-PDFT) formulations. We also consider the less affordable XMS-CASPT2 method with the same state-averaged-complete-active-space (SA-CASSCF) reference wave functions, which has seven active electrons in 10 active orbitals. The calculations use state averaging over seven states and a model space that spans the seven lowest SA-CASSCF eigenvectors. We study three on-top density functionals: tPBE, tPBE0, and MC23. Vertical excitation energies, adiabatic excitation energies, and dissociation energies, along with cuts through the potential surfaces along the dissociation coordinate, were computed with the (7, 10) active space. XMS-CASPT2 and L-PDFT with the MC23 functional show consistent and reliable performance for excitation energies, closely reproducing benchmark values, and producing smooth, physically reasonable potential energy surfaces essential for nonadiabatic dynamics simulations, but they are less accurate for bond energies. The L-PDFT calculations with the tPBE functional are more accurate for dissociation energies, but less accurate for excitation energies.

Keywords: chemistry; computer simulation; quantum chemistry; quantum mechanics; quantum science and technology; radical reactions; spectroscopy
Corresponding author: Donald G. Truhlar, Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455-0431, USA, e-mail:
Article note: A collection of invited papers to celebrate the UN’s proclamation of 2025 as the International Year of Quantum Science and Technology.

Funding source: Air Force Office of Scientific Research

Award Identifier / Grant number: FA9550-20-1-0360

Acknowledgments

The authors are grateful to Yinan Shu, Matthew Hennefarth, Matthew Hermes, and Laura Gagliardi for valuable discussions.

  1. Research ethics: Not applicable.

  2. Informed consent: Not applicable.

  3. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  4. Use of Large Language Models, AI and Machine Learning Tools: None declared.

  5. Conflict of interest: The authors declare no conflicts.

  6. Research funding: This work was supported in part by the Air Force Office of Scientific Research by grant FA9550-20-1-0360.

  7. Data availability: All data generated or analyzed during this study are included in this published article and its Supplementary information file.

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/pac-2025-0546).

Received: 2025-06-15
Accepted: 2025-08-14
Published Online: 2025-08-27
Published in Print: 2025-10-27

© 2025 IUPAC & De Gruyter

Articles in the same Issue

  1. Frontmatter
  2. Review Articles
  3. Minimum energy path methods and reactivity for enzyme reaction mechanisms: a perspective
  4. The quantum revolution in enzymatic chemistry: combining quantum and classical mechanics to understand biochemical processes
  5. A quantum chemical perspective of photoactivated biological functions
  6. Does chemistry need more physics?
  7. Rotational dynamics of ATP synthase: mechanical constraints and energy dissipative channels
  8. Transforming dreams into reality: a fairy-tale wedding of chemistry with quantum mechanics
  9. The quantum chemistry revolution and the instrumental revolution as evidenced by the Nobel Prizes in chemistry
  10. Influence of symmetry on the second-order NLO properties: insights from the few state approximations
  11. The dichotomy between chemical concepts and numbers after almost 100 years of quantum chemistry: conceptual density functional theory as a case study
  12. How ‘de facto variational’ are fully iterative, approximate iterative, and quasiperturbative coupled cluster methods near equilibrium geometries?
  13. Electronic structure of methyl radical photodissociation
  14. Bridging experiment and theory: a computational exploration of UMG-SP3 dynamics
  15. Research Articles
  16. O–Li⋯O and C–Li⋯C lithium bonds in small closed shell and open shell systems as analogues of hydrogen bonds
  17. Metal–ligand bonding and noncovalent interactions of mutated myoglobin proteins: a quantum mechanical study
https://doi.org/10.1515/pac-2025-0546
Keywords for this article
chemistry; computer simulation; quantum chemistry; quantum mechanics; quantum science and technology; radical reactions; spectroscopy

Articles in the same Issue

  1. Frontmatter
  2. Review Articles
  3. Minimum energy path methods and reactivity for enzyme reaction mechanisms: a perspective
  4. The quantum revolution in enzymatic chemistry: combining quantum and classical mechanics to understand biochemical processes
  5. A quantum chemical perspective of photoactivated biological functions
  6. Does chemistry need more physics?
  7. Rotational dynamics of ATP synthase: mechanical constraints and energy dissipative channels
  8. Transforming dreams into reality: a fairy-tale wedding of chemistry with quantum mechanics
  9. The quantum chemistry revolution and the instrumental revolution as evidenced by the Nobel Prizes in chemistry
  10. Influence of symmetry on the second-order NLO properties: insights from the few state approximations
  11. The dichotomy between chemical concepts and numbers after almost 100 years of quantum chemistry: conceptual density functional theory as a case study
  12. How ‘de facto variational’ are fully iterative, approximate iterative, and quasiperturbative coupled cluster methods near equilibrium geometries?
  13. Electronic structure of methyl radical photodissociation
  14. Bridging experiment and theory: a computational exploration of UMG-SP3 dynamics
  15. Research Articles
  16. O–Li⋯O and C–Li⋯C lithium bonds in small closed shell and open shell systems as analogues of hydrogen bonds
  17. Metal–ligand bonding and noncovalent interactions of mutated myoglobin proteins: a quantum mechanical study
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