Multiconfigurational Second-Order Perturbation Theory Restricted Active Space (RASPT2) Method for Electronic Excited...

Area of Science:

• Computational Quantum Chemistry
• Theoretical Spectroscopy
• Electronic Excited States

Background:

• Multiconfigurational perturbation theory is crucial for accurately describing electronic excited states.
• Existing methods like Complete Active Space Second-Order Perturbation Theory (CASPT2) have limitations in system size and complexity.
• Accurate computation of excited states is vital for understanding photochemistry and photophysics.

Purpose of the Study:

• To benchmark the newly developed Restricted Active Space Second-Order Perturbation Theory (RASPT2) method.
• To assess the accuracy of RASPT2 and multistate (MS) RASPT2 for computing vertical excitation energies.
• To extend the applicability of multiconfigurational perturbation theory to larger and more complex molecular systems.

Main Methods:

• Calculated vertical excitation energies for valence and Rydberg states of various organic and inorganic molecules.
• Employed RASPT2 and MS-RASPT2 methods with different reference spaces.
• Compared results against CASPT2, Coupled Cluster with Singles and Doubles (CCSD), and experimental data.

Main Results:

• RASPT2 demonstrates good accuracy in calculating excitation energies, comparable to established methods.
• The study addressed challenges like valence-Rydberg mixing in organic molecules and d-shell effects in transition metals.
• RASPT2 successfully extends the scope of accurate excited-state calculations to larger systems.

Conclusions:

• RASPT2 is a reliable and accurate method for computing excited-state properties.
• This method significantly broadens the applicability of high-level quantum chemical calculations for complex systems.
• The findings pave the way for more in-depth studies of molecular excited states in challenging systems.