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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids

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Applying Generalized Variational Principles to Excited-State-Specific Complete Active Space Self-consistent Field

Rebecca Hanscam¹, Eric Neuscamman^1,2

¹Department of Chemistry, University of California, Berkeley, California 94720, United States.

Journal of Chemical Theory and Computation

|October 10, 2022

Summary

This study introduces a new method to enhance excited-state calculations, improving stability and accuracy for complex molecules. The generalized variational principle offers better convergence and state targeting than previous techniques.

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Area of Science:

Quantum Chemistry
Computational Chemistry
Theoretical Chemistry

Background:

Accurate calculation of excited electronic states is crucial for understanding molecular properties and reactions.
Complete Active Space Self-Consistent Field (CASSCF) theory is a standard method for excited states, but can suffer from convergence and stability issues.
Existing methods for excited-state targeting often struggle with root flipping and achieving precise stationary points.

Purpose of the Study:

To develop a generalized variational principle for improving the stability, reliability, and precision of excited-state-specific CASSCF theory.
To enable optimal orbital shapes for individual excited states, particularly for challenging cases like charge-transfer and doubly excited states.
To demonstrate the method's ability to find multiple excited-state stationary points, including those previously inaccessible.

Main Methods:

Employing a generalized variational principle within the CASSCF framework.
Tailoring orbitals and configuration interaction expansion for individual excited states.
Testing the method on LiH, ozone, and MgO molecules.

Main Results:

The new approach shows increased resistance to root flipping compared to previous methods.
The method achieves tighter convergence to excited-state energy stationary points.
Demonstrated successful state-targeting for individual excited states, crucial for charge-transfer and doubly excited states.
Successfully located three excited-state stationary points in MgO, a feat not achieved by prior methods.

Conclusions:

The generalized variational principle significantly enhances the performance of excited-state-specific CASSCF theory.
This method provides a more robust and accurate tool for computational investigations of excited electronic states.
The improved state-targeting capabilities open new avenues for studying complex electronic phenomena in molecules.