Internally contracted multireference coupled-cluster theory in a multistate framework

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• The internally contracted multireference coupled cluster (icMRCC) theory is a powerful method for describing strongly correlated electronic systems.
• Existing frameworks often struggle with accurately describing systems near avoided crossings.

Purpose of the Study:

• To reexamine the internally contracted multireference coupled cluster (icMRCC) theory within a multistate framework.
• To generalize state-universal multireference coupled-cluster (SU-MRCC) theory.
• To investigate and resolve artifacts in truncated icMRCC theory.

Main Methods:

• Derivation of working equations from Bloch equations using a wave operator.
• Adaptation of the wave operator for a multideterminantal model-space basis.
• Comparison of state-specific icMRCC solutions with multistate theory.

Main Results:

• The working equations generalize state-universal multireference coupled-cluster (SU-MRCC) theory.
• State-specific icMRCC solutions decouple the multistate equations by diagonalizing the effective Hamiltonian.
• Truncated icMRCC theory exhibits minor artifacts near avoided crossings, resolvable with a true multistate ansatz.

Conclusions:

• icMRCC theory is fundamentally a multistate theory.
• A true multistate ansatz is necessary to fully resolve artifacts near avoided crossings.
• The study provides new insights into the orbital variance of SU-MRCC theory.