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A new microcanonical instanton theory accurately describes quantum tunneling in chemical reactions, improving upon existing methods for complex systems and deep-tunneling regimes.

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Area of Science:

  • Quantum Chemistry
  • Chemical Dynamics
  • Theoretical Chemistry

Background:

  • Canonical (thermal) instanton theory accurately describes tunneling in complex gas-phase reactions.
  • Microcanonical instanton theory is less established and has limitations.

Purpose of the Study:

  • To address the limitations of existing microcanonical instanton theory.
  • To develop an improved microcanonical instanton theory for accurate quantum tunneling description.

Main Methods:

  • A first-principles derivation of microcanonical instanton theory.
  • Integration over energy to recover thermal instanton rates.
  • Combination with the density-of-states approach.

Main Results:

  • The established microcanonical theory fails in deep-tunneling regimes with rapidly changing orthogonal mode frequencies.
  • The improved microcanonical instanton theory accurately recovers thermal instanton rates.
  • The new theory correctly reproduces the separable limit.

Conclusions:

  • The developed microcanonical instanton theory is a generalization of Rice-Ramsperger-Kassel-Marcus (RRKM) theory.
  • This improved method is applicable to real molecular systems when combined with density-of-states.
  • Accurate description of quantum tunneling in complex chemical systems is achieved.