MR-MCTDH[n]: Flexible Configuration Spaces and Nonadiabatic Dynamics within the MCTDH[n] Framework

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Solving the time-dependent Schrödinger equation (TDSE) for large molecules is computationally intensive due to exponential scaling.
• The multiconfiguration time-dependent Hartree (MCTDH) method provides accurate solutions for small to medium systems.
• Previous MCTDH[n] approximations addressed scaling but struggled with systems where specific degrees of freedom are crucial.

Purpose of the Study:

• To introduce a multireference extension (MR-MCTDH[n]) to the MCTDH[n] methods.
• To enable more accurate and efficient quantum dynamics simulations for complex molecular systems.
• To improve the treatment of systems with special degrees of freedom, such as double-well potentials or nonadiabatic processes.

Main Methods:

• Development of the multireference multiconfiguration time-dependent Hartree [n] (MR-MCTDH[n]) method.
• Inclusion of selected higher-order excitations for specific degrees of freedom within the MCTDH[n] framework.
• Application to model systems: formyl fluoride (intramolecular vibrational-energy redistribution), salicylaldimine (double-well dynamics), and pyrazine (nonadiabatic dynamics).

Main Results:

• The MR-MCTDH[n] method successfully extends the applicability of MCTDH[n] to larger and more complex molecular systems.
• Efficient treatment of nonadiabatic dynamics is achieved by describing wave packets on different electronic surfaces with a consistent approximation level.
• Fast convergence is observed when the configuration space is appropriately extended for the special modes governing quantum dynamics.

Conclusions:

• The MR-MCTDH[n] method offers a flexible and efficient approach for simulating quantum dynamics in challenging molecular systems.
• This advancement is particularly beneficial for studying phenomena like nonadiabatic transitions, IVR, and double-well dynamics.
• The findings demonstrate the power of multireference extensions in overcoming the limitations of single-reference methods in quantum dynamics.