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.