Accurate Equilibrium Structures of Organic Radicals Made Accessible with Local-Correlation-Enhanced Pisa Composite...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Chemical Physics

Background:

• Accurate equilibrium structures of organic radicals are limited, hindering progress in atmospheric chemistry, combustion, spectroscopy, and catalysis.
• Experimental determination of radical structures is often inaccessible, necessitating reliable quantum-chemical methods.
• Existing double-hybrid density functionals struggle with delocalized open-shell systems and may require problematic bond-length corrections.

Purpose of the Study:

• To develop a robust and practical quantum-chemical protocol for accurate equilibrium structures of organic radicals.
• To address the limitations of current methods in describing delocalized open-shell systems.
• To provide a broadly accessible computational strategy for high-precision radical geometries.

Main Methods:

• Utilized a local correlation treatment based on pair natural orbitals (PNOs).
• Employed the PNO-LCCSD(F12b)(T*) approach within an efficient composite framework.
• Developed the PPCS2 protocol by combining DFT geometries/Hessians with local correlation, hierarchical optimization, and an efficient driver in generalized internal coordinates.

Main Results:

• Achieved near-spectroscopic equilibrium geometries for both closed- and open-shell molecular systems up to a few dozen atoms.
• Demonstrated uniform accuracy across diverse radical types (σ, π, aromatic, heteroatom-centered).
• Delivered high-precision equilibrium structures for challenging reactive radicals.

Conclusions:

• The PNO-based local correlation treatment offers a robust solution for determining accurate radical equilibrium structures.
• The developed PPCS2 protocol enables automated assembly of composite gradients and broad accessibility.
• This approach significantly advances the ability to characterize reactive radicals computationally.