Multi-d-Occupancy as an Alternative Definition for the Double d-Shell Effect

Area of Science:

• Theoretical Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• First-row transition metal compounds are vital in chemistry but difficult to model accurately.
• Challenges include defining ground-state multiplicities and convergence issues in excited-state calculations.
• Existing multireference methods struggle with the double d-shell effect in these systems.

Purpose of the Study:

• To explore the double d-shell effect impacting multireference calculations for transition metal systems.
• To propose an alternative definition of the double d-shell effect based on multi-d-occupancy character.
• To develop a protocol for incorporating this effect into multireference calculations with smaller active spaces.

Main Methods:

• Proposed an alternative definition of the double d-shell effect.
• Developed a protocol to include this effect in multireference calculations.
• Utilized a molybdenum-copper model system and its copper subsystem for case studies.

Main Results:

• Demonstrated the utility of the alternative definition for qualitative wave function analysis.
• Showed that optimizing active space based on this definition yields accurate relative energies.
• Successfully reduced computational cost while maintaining accuracy.

Conclusions:

• The proposed multi-d-occupancy character definition improves wave function description and relative energy accuracy.
• This approach offers a potential pathway to better theoretical modeling of transition metal compounds.
• The protocol can enhance accuracy and reduce computational expense for ground and excited electronic structure calculations.