Conical Intersections in Solution with Polarizable Embedding: Integral-Exact Direct Reaction Field.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Mechanics/Molecular Mechanics (QM/MM)

Background:

• QM/MM embedding is crucial for studying complex systems, particularly electronic excited states.
• Polarizable embedding (QM/MM-Pol) is state-of-the-art but suffers from root flipping and incorrect electronic crossing descriptions.
• Accurate treatment of QM/MM coupling is vital for charge-transfer excited states.

Purpose of the Study:

• To address limitations in current QM/MM-Pol methods, specifically root flipping and inaccurate electronic state crossings.
• To develop a robust QM/MM embedding approach for reliable multistate calculations.
• To investigate the influence of a polarizable environment on photochemical processes.

Main Methods:

• Integral-exact reformulation of the direct reaction field (DRF) approach (QM/MM-IEDRF).
• Inclusion of one- and two-electron operators and many-body dipole-induced dipole interactions.
• Mimicking Pauli repulsion using effective core potentials and accounting for instantaneous MM dipole response.

Main Results:

• The QM/MM-IEDRF method provides a consistent description of electronic states and their crossings.
• It naturally incorporates screening of electron-electron interactions by MM induced dipoles.
• Demonstrated application to ethylene photoisomerization in a xenon matrix, analyzing conical intersections.

Conclusions:

• QM/MM-IEDRF offers a significant improvement over existing QM/MM-Pol methods for excited-state calculations.
• The approach ensures consistent definition of transition properties and state crossings.
• This method enables accurate exploration of environmental effects on photochemical reactions.