Description of electronic excited states using electron correlation operator.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Electron correlation energy is crucial for accurate chemical system energy calculations.
• Existing methods often rely on approximations like mean-field theory.
• A universal, position-independent correlation operator is desirable for practical applications.

Purpose of the Study:

• To develop and test a novel, universal linear two-electron correlation operator.
• To model electron correlation effects using an operator expanded in Gaussians.
• To compute atomic and molecular adiabatic excited states and compare with existing methods.

Main Methods:

• Utilized a linear two-electron operator for Kohn-Sham wavefunctions.
• Employed the Δ Self-Consistent Field (ΔSCF) formalism for excited state computations.
• Compared results with time-dependent density functional theory (TD-DFT) using popular functionals.

Main Results:

• The correlation operator approach significantly outperforms other methods for atomic excitation energies.
• The simplest correlation operator form, parameterized by helium atom data, showed strong performance.
• Accuracy for molecular excitation energies was constrained by ΔSCF limitations in describing excited states.

Conclusions:

• The developed correlation operator is a promising tool for modeling electron correlation, especially for atomic systems.
• Further development is needed to overcome ΔSCF limitations for accurate molecular excited state predictions.
• This approach offers a computationally efficient alternative for certain quantum chemical calculations.