Performance Tests of the Second-Order Approximate Internally Contracted Multireference Coupled-Cluster Singles and...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The single-reference coupled-cluster method CC2 is widely used but has limitations for systems with strong multi-reference character.
• Developing accurate and efficient multireference methods is crucial for understanding complex electronic structures.

Purpose of the Study:

• To benchmark the performance of the internally contracted multireference coupled-cluster method with single and double excitations (icMRCC2).
• To assess the accuracy of icMRCC2 for calculating vertical excitation energies, spectroscopic constants, and geometric parameters.
• To demonstrate the advantages of icMRCC2 over single-reference CC2 for challenging chemical systems.

Main Methods:

• Application of the icMRCC2 method to various small to medium-sized organic molecules and diatomics.
• Comparison of icMRCC2 results with established literature data and experimental values.
• Investigation of the impact of active space size on icMRCC2 accuracy.

Main Results:

• icMRCC2 accurately predicts vertical excitation energies, spectroscopic constants, and geometric parameters.
• The method overcomes limitations of CC2, including issues with Rydberg excitations and multiple bonds.
• Accurate results can often be obtained with small active spaces in icMRCC2.

Conclusions:

• icMRCC2 offers significant improvements in accuracy over standard multireference perturbation theories.
• The method extends the applicability of CC2 to systems with strong multi-reference character.
• icMRCC2 shows great potential for studying complex chemical phenomena, including automerization and excited states of transition metal complexes.