CASSCF linear response calculations for large open-shell molecules.

Area of Science:

• Computational Chemistry
• Theoretical Spectroscopy
• Quantum Chemistry

Background:

• Simulating UV/Vis absorption and ECD spectra is crucial for understanding molecular electronic properties.
• Accurate spectral simulations for large open-shell molecules present significant computational challenges.
• Existing methods like state-averaged CASSCF may lack sufficient accuracy for complex systems.

Purpose of the Study:

• To present a complete active space self-consistent-field (CASSCF) linear response method for simulating UV/Vis and ECD spectra.
• To enhance computational efficiency for large open-shell molecules.
• To improve the accuracy of spectral simulations compared to previous methods.

Main Methods:

• Utilized a one-index transformed Hamiltonian for integral-direct computation of intermediates.
• Employed resolution-of-the-identity and overlap-fitted chain-of-spheres approximations.
• Implemented iterative diagonalization with Hermitian and anti-Hermitian trial vectors for efficient eigenvalue problem solutions.

Main Results:

• Developed and implemented an efficient CASSCF linear response method.
• Introduced natural transition orbitals for characterizing electronic transitions within the CASSCF linear response framework.
• Demonstrated significantly improved accuracy for simulated UV/Vis spectra of an organic radical compared to state-averaged CASSCF.
• Successfully computed spectra for a large Ni triazole complex (231 atoms), demonstrating computational efficiency.

Conclusions:

• The presented CASSCF linear response method provides an accurate and efficient tool for simulating UV/Vis and ECD spectra of large open-shell molecules.
• The method's efficiency is suitable for tackling complex molecular systems.
• Natural transition orbitals offer valuable insights into electronic transitions.