Excitation energies from particle-particle random phase approximation with accurate optimized effective potentials

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Accurate calculation of electronic excited states is crucial for understanding molecular properties and reactions.
• Kohn-Sham density functional theory (DFT) provides a framework for electronic structure calculations, but obtaining accurate excited states can be challenging.
• The optimized effective potential (OEP) method offers a way to derive accurate Kohn-Sham (KS) orbitals and energies from a reference electron density.

Purpose of the Study:

• To investigate the accuracy of electronic excited state calculations using the particle-particle random phase approximation (pp-RPA) method.
• To evaluate the performance of OEP-derived KS orbitals and energies, obtained from various reference electron densities, in pp-RPA calculations.
• To determine the theoretical limit of pp-RPA excitation energies when using exact KS-DFT orbitals and orbital energies.

Main Methods:

• Calculated electronic excited states using the particle-particle random phase approximation (pp-RPA).
• Employed optimized effective potential (OEP) to obtain accurate Kohn-Sham (KS) orbitals and orbital energies from reference electron densities.
• Investigated various input densities, including those from coupled-cluster singles and doubles (CCSD) methods, for OEP calculations.

Main Results:

• pp-RPA excitation energies were calculated using exact KS-DFT orbitals and orbital energies.
• The use of OEP with electron densities from coupled-cluster singles and doubles (CCSD) methods resulted in the lowest mean absolute error for low-lying excited states.
• The study provides insights into the theoretical limits of pp-RPA for excitation energy calculations.

Conclusions:

• OEP-derived KS orbitals and energies, particularly those from CCSD densities, significantly improve the accuracy of pp-RPA excited state calculations.
• The findings highlight the importance of accurate input densities for reliable excitation energy predictions.
• Higher-order correlation contributions beyond the pp-RPA Coulomb kernel may be necessary for achieving even greater accuracy in future calculations.