Multireference Algebraic Diagrammatic Construction Theory for Excited States: Extended Second-Order Implementation...

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Theoretical Molecular Physics

Background:

• Accurate simulation of electronic excitations and UV/vis spectra is crucial for understanding molecular properties.
• Strongly correlated molecular systems pose significant challenges for traditional quantum chemical methods.
• Existing multireference perturbation theories often lack efficient excited-state property calculations.

Purpose of the Study:

• To implement and benchmark new second-order approximations for multireference algebraic diagrammatic construction (MR-ADC) theory.
• To accurately simulate neutral electronic excitations and UV/vis spectra of strongly correlated molecular systems.
• To provide efficient methods for calculating excited-state properties without state-averaged reference wave functions.

Main Methods:

• Development and implementation of strict and extended second-order MR-ADC methods (MR-ADC(2) and MR-ADC(2)-X).
• Combination of static and dynamic electron correlation for ground and excited electronic states.
• Benchmarking on small molecules like carbon dimer, ethylene, and butadiene for excited states.

Main Results:

• MR-ADC(2) and MR-ADC(2)-X methods show strong performance for weakly correlated states, outperforming third-order single-reference ADC.
• Results are competitive with equation-of-motion coupled cluster theory for weakly correlated states.
• Performance for multireference character states is comparable to N-electron valence perturbation theory.

Conclusions:

• The new MR-ADC methods effectively capture static and dynamic electron correlation for excited states.
• These methods offer an efficient and straightforward approach to calculating excited-state properties.
• MR-ADC provides direct access to excitations beyond active orbitals, a significant advantage over conventional methods.