Communication: multireference equation of motion coupled cluster: a transform and diagonalize approach to electronic...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Accurate calculation of electronic states is crucial for understanding molecular properties and reactions.
• Traditional methods often struggle with systems exhibiting strong electron correlation or requiring a multireference description.
• Developing efficient and accurate quantum chemical methods remains a key challenge.

Purpose of the Study:

• To introduce and validate novel multireference equation-of-motion coupled-cluster (MREOM-CC) approaches.
• To demonstrate the versatility and accuracy of MREOM-CC for calculating a large number of electronic states.
• To assess the computational efficiency and applicability of MREOM-CC to complex chemical systems.

Main Methods:

• The study employs a sequence of many-body similarity transformations to a compact subspace.
• A non-Hermitean transformed Hamiltonian is diagonalized, preserving spin and spatial symmetries.
• The diagonalization space is defined by a complete active space (CAS) with limited excitations, ensuring invariance to orbital rotations.

Main Results:

• MREOM-CC methods provide accurate access to a large number of electronic states, with applications to Cr, Mn, and Fe atoms yielding up to 524 states.
• An root-mean-square error of approximately 0.05 eV compared to experimental data was achieved for transition metal atoms.
• Favorable extensivity properties were demonstrated through calculations on the O2-O2 dimer, highlighting the method's scalability.
• Computational costs for MREOM-CC transformation steps are comparable to the Coupled Cluster Singles and Doubles (CCSD) approach.

Conclusions:

• MREOM-CC methods represent a significant advancement in accurately calculating electronic states for complex systems.
• The demonstrated accuracy and efficiency make MREOM-CC a powerful tool for theoretical chemistry research.
• These methods are well-suited for studying systems with multireference character, including transition metal atoms and molecular complexes.