Spin-Adapted Externally Contracted Multireference Configuration Interaction Method Based on Selected Reference...

Area of Science:

• Quantum Chemistry
• Computational Chemistry

Background:

• Selected configuration interaction (sCI) is an approximation for full configuration interaction, valuable for large active spaces.
• Dynamic correlation is crucial for quantitative results beyond large active spaces.
• Externally contracted multireference configuration interaction (EC-MRCI) using sCI references is promising but has drawbacks.

Purpose of the Study:

• To address spin contamination and low efficiency in previous sCI-EC-MRCI methods.
• To develop a more efficient and accurate computational approach for large quantum systems.

Main Methods:

• Utilized configuration state function (CSF) bases instead of Slater determinants.
• Introduced a hybrid algorithm combining tree structures for configuration space management and the graphical unitary group approach (GUGA) for matrix element calculation.
• Developed a spin-adapted version of the method.

Main Results:

• The spin-adapted CSF-based method achieved a 6.0 speed-up compared to the Slater determinant version for naphthalene.
• The improved sCI-EC-MRCI method provided quantitatively accurate results for dinuclear copper(II) compounds and Ln(III)/An(III) complexes.
• Demonstrated the ability to include dynamic correlation beyond sCI for large active spaces and basis sets.

Conclusions:

• The new CSF-based sCI-EC-MRCI method significantly improves efficiency and reduces spin contamination.
• This approach enables accurate treatment of dynamic correlation in complex systems with large active spaces and basis sets.
• The developed computational strategy is effective for challenging chemical systems in quantum chemistry.