Approaching the Complete Basis Set Limit for Spin-State Energetics of Mononuclear First-Row Transition Metal...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Inorganic Chemistry

Background:

• Accurate prediction of spin-state energetics is crucial for understanding transition metal (TM) complexes.
• Achieving the complete basis set (CBS) limit is computationally demanding for these systems.

Purpose of the Study:

• To analyze the convergence to the CBS limit for spin-state energetics in mononuclear first-row TM complexes.
• To develop an economical computational protocol for accurate spin-state energy calculations.

Main Methods:

• Systematic study of coupled cluster with singles and doubles (CCSD(T)) and explicitly correlated CCSD(T)-F12a/b calculations.
• Development of a protocol using CCSD-F12a and modified scaling of the perturbative triples term (T#).
• Validation against a benchmark set of 18 spin-state energy differences for 13 TM complexes.

Main Results:

• The developed protocol accurately reproduces reference CCSD(T)/CBS spin-state energetics (MAD=0.4 kcal/mol).
• The protocol allows for canonical CCSD(T) calculations on complexes up to ~50 atoms, demonstrated with metalloporphyrins.
• Basis set incompleteness error (BSIE) is transferable across various wave function methods.

Conclusions:

• An efficient and accurate computational protocol for spin-state energetics in TM complexes has been established.
• The findings facilitate larger-scale computational studies and simplify multi-method benchmark studies.
• The transferability of BSIE supports the use of focal-point approximations.