Dissipative dynamics at conical intersections: simulations with the hierarchy equations of motion method

Area of Science:

• * Physical Chemistry
• * Quantum Dynamics
• * Computational Chemistry

Background:

• * Conical intersections are crucial for ultrafast nonadiabatic dynamics in molecules.
• * Understanding the influence of dissipative environments (the bath) on these dynamics is essential.
• * Previous studies were limited to weak system-bath coupling regimes.

Purpose of the Study:

• * To analyze the effect of a dissipative environment on ultrafast nonadiabatic dynamics at conical intersections.
• * To model the S2(ππ*)–S1(nπ*) conical intersection in pyrazine coupled to a harmonic bath.
• * To extend simulations to arbitrary system-bath coupling strengths and bath memory times.

Main Methods:

• * A two-state, two-mode model representing the pyrazine conical intersection.
• * System-bath coupling modeled using the Drude spectral function.
• * Numerical exact solution of the reduced density matrix using the hierarchy equation of motion method with GPU acceleration.

Main Results:

• * Weak system-bath coupling effectively suppresses electronic population fluctuations.
• * The upper adiabatic state (S2) population is efficiently quenched.
• * The diabatic ππ* state population shows long-lived oscillations driven by the tuning mode.
• * System-bath coupling can enhance tuning mode coherence by damping the coupling mode.

Conclusions:

• * The study extends the understanding of dissipative dynamics at conical intersections into the nonperturbative regime.
• * Results demonstrate counterintuitive effects of bath coupling on electronic and vibrational coherence.
• * Paves the way for first-principles simulations of complex molecular dynamics in condensed phases.