Pyrazine excited states revisited using the extended multi-state complete active space second-order perturbation...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Standard multi-state complete active space second-order perturbation theory (MS-CASPT2) methods exhibit limitations in describing potential energy surfaces near crossing points.
• Accurate modeling of excited states is crucial for understanding molecular photophysics and photochemistry.

Purpose of the Study:

• To evaluate the performance of the extended multi-state complete active space second-order perturbation theory (XMS-CASPT2) for excited singlet states of pyrazine.
• To compare XMS-CASPT2 with standard MS-CASPT2 and multireference configuration interaction (MRCI) methods.

Main Methods:

• Utilized extended multi-state complete active space second-order perturbation theory (XMS-CASPT2).
• Employed a vibronic-coupling model Hamiltonian for spectral simulations.
• Ensured method invariance under unitary rotations of reference functions for artifact-free conical intersection description.

Main Results:

• XMS-CASPT2 yields qualitatively correct potential energy surfaces for pyrazine's low-lying excited singlet states, overcoming issues seen in standard MS-CASPT2.
• Simulated photoabsorption spectra for the 1(1)B(3u) and 1(1)B(2u) states show good agreement with experimental data and MRCI calculations.
• XMS-CASPT2 performance for the 1(1)B(3u) band is comparable to the more computationally expensive MRCI method.

Conclusions:

• XMS-CASPT2 is a reliable and efficient method for studying excited states and conical intersections in molecules like pyrazine.
• The method offers a good balance between accuracy and computational cost for photochemical applications.