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

In this work, we implement a local pair natural orbital-based coupled-cluster method through the full treatment of quadruple excitations (CCSDTQ). The domain-based local pair natural orbital (DLPNO) approach, which has successfully been applied to lower levels of coupled-cluster theory, is utilized in our algorithm, and thus our algorithm is called DLPNO-CCSDTQ. For simplicity in the working equations and in the implementation, we t₁-dress the two-electron integrals as well as Fock matrix elements. Our method can recover CCSDTQ-CCSDT and CCSDTQ-CCSDT(Q) energy differences on the order of 0.01-0.05 kcal mol^-1, even at a loose quadruples natural orbital (QNO) occupation number cutoff of 3.33 × 10^-6. To highlight the capabilities of our code and its potential future applications, we showcase computations that would be intractable with canonical CCSDTQ, such as the benzene dimer, (H₂O)₁₇, and adamantane. With sufficient computing resources, computations up to 15 heavy atoms (40 atoms overall) may be feasible for fully bonded 3D systems.