Partially linearized external models to active-space coupled-cluster through connected hextuple excitations.

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Full active-space calculations can be computationally prohibitive.
• Approximations are needed to reduce the cost of high-level CC methods.

Purpose of the Study:

• To implement and compare partially linearized external models for active-space coupled-cluster.
• To evaluate the accuracy and computational efficiency of these approximate models.
• To investigate molecular properties like bond dissociation energies and potential energy surfaces.

Main Methods:

• Implementation of partially linearized external models: CC(SDtqph)_L, CCSD(tqph)_L, and CCSD(tqph)_hyb.
• Comparison with the full active-space CCSD(tqph) model.
• Application to diatomic molecules (HF, F₂, CuH, CuF) and potential energy surfaces (HF, CuH, H₂O, C₂H₄).

Main Results:

• The computational scaling of CCSD(tqph) is similar to CCSD but with a larger prefactor.
• Approximate linearization schemes are significantly cheaper than full CCSD(tqph).
• CCSD(tqph)_hyb demonstrates highly accurate results comparable to the full CCSD(tqph) model across all tested systems.

Conclusions:

• Partially linearized external models provide a computationally feasible approach to high-level coupled-cluster theory.
• The CCSD(tqph)_hyb model offers an excellent balance between accuracy and computational cost.
• These methods are valuable tools for studying chemical reaction pathways and molecular energetics.