Ab initio theory for treating local electron excitations in molecules and its performance for computing optical...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Predicting molecular electronic excited state properties computationally is crucial for understanding molecular behavior.
• Ab initio methods offer high accuracy but can be computationally expensive.

Purpose of the Study:

• To develop an efficient approximation for predicting molecular electronic excited state properties at the ab initio level.
• Introduce and validate the Local Excitation Approximation (LEA) scheme.

Main Methods:

• The Local Excitation Approximation (LEA) scheme is proposed, focusing on local electron excitations within a selected chromophore.
• Localized molecular orbitals (LMOs) are used to define the chromophore substructure.
• Davidson's iterative diagonalization convergence issues are addressed by transforming LMOs.

Main Results:

• The LEA scheme can accurately predict excited state properties when transitions have strong local character and an appropriate chromophore is chosen.
• Computational efficiency is improved by discarding non-local electron excitations.
• Test calculations using Configuration Interaction Single (CIS) and Time-Dependent Hartree-Fock (TDHF) levels of theory were performed.

Conclusions:

• The LEA scheme provides a promising approach for efficient and accurate ab initio calculations of molecular electronic excited states.
• This method can reduce computational cost without significant loss of accuracy for systems with localized excitations.