High-order excitations in state-universal and state-specific multireference coupled cluster theories: model systems

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster (CC) methods are essential for accurate electronic structure calculations.
• Multireference coupled cluster (MRCC) theories are needed for systems with complex electronic structures.
• High-order excitations are crucial for describing electron correlation accurately.

Purpose of the Study:

• To investigate high-order excitations (n>2) in three distinct MRCC theories.
• To compare the performance of state-universal (SU) and state-specific (Brillouin-Wigner, BW; Mukherjee, Mk) MRCC methods.
• To assess the accuracy of MRCC methods against full configuration interaction (FCI) and single-reference CC.

Main Methods:

• Development and application of a new arbitrary-order string-based code for MRCC calculations.
• Computation of MRCC wave functions with excitations up to hextuples.
• Application to model systems: H4, P4, BeH(2), and H8.

Main Results:

• BW and Mk MRCC methods generally provide more accurate ground-state results than the SU approach across truncation levels.
• Inclusion of connected triple excitations reduces nonparallelism error in singles and doubles MRCC energies by 2-10 times.
• Including all quadruple excitations in BeH(2) and H8 models yields energies within 1 kcal mol(-1) of the FCI limit.

Conclusions:

• State-specific MRCC methods (BW and Mk) are effective for multireference systems.
• Single-reference CC methods are superior when a single electronic configuration dominates.
• High-order excitations significantly improve the accuracy of MRCC calculations, approaching FCI limits.