Local Pair Natural Orbital-Based Coupled-Cluster Theory through Full Quadruples (DLPNO-CCSDTQ)

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster theory is a powerful tool for accurate electronic structure calculations.
• Full treatment of quadruple excitations (CCSDTQ) provides high accuracy but is computationally expensive.
• Local correlation methods, like DLPNO, reduce computational cost by exploiting locality.

Purpose of the Study:

• To implement and validate a Domain-Based Local Pair Natural Orbital Coupled Cluster method with full quadruple excitations (DLPNO-CCSDTQ).
• To assess the accuracy of DLPNO-CCSDTQ by comparing its energy differences with canonical CCSDTQ.
• To demonstrate the capability of DLPNO-CCSDTQ for large and complex molecular systems.

Main Methods:

• Implementation of a local pair natural orbital (LPNO) based coupled-cluster (CC) method.
• Full treatment of quadruple excitations within the CC ansatz.
• Utilizing the domain-based LPNO (DLPNO) approach for computational efficiency.
• Employing t1-dressed two-electron integrals and Fock matrix elements for simplified working equations.

Main Results:

• DLPNO-CCSDTQ recovers CCSDTQ-CCSDT and CCSDTQ-CCSDT(Q) energy differences within 0.01-0.05 kcal mol⁻¹.
• The method remains accurate even with a loose quadruples natural orbital (QNO) occupation number cutoff.
• Successful application to systems intractable for canonical CCSDTQ, including benzene dimer, (H₂O)₁₇, and adamantane.

Conclusions:

• DLPNO-CCSDTQ offers a computationally feasible approach to high-accuracy coupled-cluster calculations.
• The developed method significantly expands the scope of problems addressable by CCSDTQ-level theory.
• Future applications may include systems with up to 15 heavy atoms (40 atoms overall) given adequate computational resources.