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

## TL;DR

This paper introduces a new quantum chemistry method for accurately calculating molecular energies using advanced computational techniques.

## Contribution

The novel method extends local pair natural orbital-based coupled-cluster theory to include full quadruples for higher accuracy.

## Key findings

- The method achieves energy differences within 0.01–0.05 kcal mol–1 compared to canonical CCSDTQ.
- Computations for complex systems like benzene dimer and adamantane become feasible with this approach.

## Abstract

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 t
1-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.