Analytic gradient and derivative couplings for the spin-flip extended configuration interaction singles method:...

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Accurate excited-state calculations are crucial for understanding photophysical and photochemical processes.
• Existing methods can be computationally expensive for large molecular systems.
• Describing conical intersections is essential for modeling nonadiabatic dynamics.

Purpose of the Study:

• To present the formalism of analytic gradients and derivative couplings for the spin-flip extended configuration interaction with single excitations (SF-XCIS) method.
• To report an efficient implementation of SF-XCIS within semiempirical quantum chemistry for large systems.
• To evaluate the performance of SF-XCIS with orthogonalization-corrected models (OMx) for excitation energies.

Main Methods:

• Development of analytic gradients and derivative couplings for SF-XCIS.
• Implementation of SF-XCIS in a semiempirical quantum chemistry framework.
• Statistical evaluation of SF-XCIS/OMx for vertical singlet excitation energies.
• Application in fewest switches surface hopping (FSSH) simulations for nonadiabatic dynamics.

Main Results:

• An efficient SF-XCIS implementation for fast excited-state calculations in large systems.
• Balanced treatment of ground and excited states, accurately describing conical intersections.
• Successful pilot study using OM2/SF-XCIS FSSH for excited-state proton transfer in 7-(2-pyridyl)indole.

Conclusions:

• The SF-XCIS method provides an efficient and accurate approach for excited-state calculations.
• Its ability to describe conical intersections makes it suitable for nonadiabatic dynamics simulations.
• This work demonstrates the utility of SF-XCIS in studying complex photochemical processes.