The electronic nonadiabatic coupling term: can it be ignored in dynamic calculations?

Area of Science:

• Quantum chemistry
• Chemical dynamics
• Theoretical chemistry

Background:

• Conical intersections (CIs) are critical points in molecular systems where adiabatic states become degenerate.
• Nonadiabatic coupling terms (NACTs) are essential components of the nuclear Born-Oppenheimer-Schrodinger equation, governing transitions between electronic states.
• Dynamical calculations often neglect NACTs due to computational complexity, despite their importance.

Purpose of the Study:

• To evaluate the validity of approximations that neglect Nonadiabatic Coupling Terms (NACTs) in molecular dynamics.
• To assess the performance of the semiclassical surface hopping method and the vibrational coupling model Hamiltonian in dynamics calculations.
• To determine the extent to which omitting NACTs is justified in the study of conical intersections.

Main Methods:

• Analysis of two theoretical frameworks: the semiclassical surface hopping method and the vibrational coupling model Hamiltonian.
• Examination of the role and impact of Nonadiabatic Coupling Terms (NACTs) in nuclear dynamics.
• Comparison of dynamical calculations with and without the explicit inclusion of NACTs.

Main Results:

• The study critically examines the common practice of omitting NACTs in dynamical simulations.
• It provides insights into the limitations and applicability of methods that bypass NACT calculations.
• The justification for neglecting NACTs is assessed within the context of specific theoretical models.

Conclusions:

• The findings highlight the conditions under which ignoring NACTs might be acceptable or lead to inaccuracies.
• This research contributes to a better understanding of the approximations used in simulating molecular dynamics near conical intersections.
• Recommendations are provided for the appropriate application of theoretical models in chemical dynamics.