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Tracking Excited States in Wave Function Optimization Using Density Matrices and Variational Principles.

Lan Nguyen Tran^1,2, Jacqueline A R Shea¹, Eric Neuscamman^1,3

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

Journal of Chemical Theory and Computation

|August 9, 2019

Summary

This summary is machine-generated.

This study introduces a new method for tracking individual excited states in quantum chemistry calculations, improving accuracy and stability. The approach enhances excited-state energy calculations and avoids issues in potential energy surface mapping.

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Area of Science:

Quantum Chemistry
Computational Chemistry
Theoretical Chemistry

Background:

Complete Active Space Self-Consistent Field (CASSCF) theory is crucial for accurately describing electronic structures.
Standard CASSCF methods face challenges with excited states, including root flipping and near-degeneracies.
Accurate excited-state calculations are vital for understanding chemical reactions and spectroscopy.

Purpose of the Study:

To develop a robust method for optimizing individual excited states in CASSCF theory.
To overcome common difficulties like root flipping and near-degeneracies in excited-state calculations.
To improve the accuracy and reliability of excited-state energy stationary point determination.

Main Methods:

Combines the Maximum Overlap Method (MOM) with variational principles for excited states.
Develops a state-specific orbital optimization technique to track individual electronic states.
Implements and tests the method on challenging cases including charge transfers and double excitations.

Main Results:

The proposed method effectively tracks individual excited states during orbital optimization.
Demonstrates superior performance compared to energy-based root selection and generalized MOM.
Improves the accuracy of Complete Active Space Perturbation Theory (CASPT2) for vertical excitation energies.
Avoids discontinuities in potential energy surfaces often seen with state-averaging methods.

Conclusions:

The new state-specific optimization method offers a reliable approach for excited-state calculations.
Its simplicity, low cost, and compatibility with large active spaces make it broadly applicable.
This method enhances the study of excited-state phenomena across various chemical systems.