# Near-linear Scaling in DMRG-based Tailored Coupled Clusters: An Implementation of DLPNO-TCCSD and DLPNO-TCCSD(T)

## TL;DR

This paper introduces a new, more accurate and scalable implementation of the DMRG-based tailored coupled clusters method (TCCSD) using the DLPNO approach, achieving near-linear scaling and high accuracy in complex systems.

## Contribution

The paper presents a novel DLPNO-TCCSD implementation that improves accuracy, scaling, and consistency over previous LPNO versions, including the addition of perturbative triples correction.

## Key findings

- Achieved 99.8-99.9% correlation energy retrieval in tested systems.
- Obtained energy differences within 1 kcal/mol of chemical accuracy.
- Demonstrated near-linear scaling in the new implementation.

## Abstract

We present a new implementation of DMRG-based tailored coupled clusters method (TCCSD), which employs the domain-based local pair natural orbital approach (DLPNO-TCCSD). Compared to the previous LPNO version of the method, the new implementation is more accurate, offers more favorable scaling and provides more consistent behavior across the variety of systems. On top of the singles and doubles, we include the perturbative triples correction (T), which is able to retrieve even more dynamic correlation. The methods were tested on three systems: tetramethyleneethane, oxo-Mn(Salen) and Iron(II)-porphyrin model. The first two were revisited to assess the performance with respect to LPNO-TCCSD. For oxo-Mn(Salen), we retrieved between 99.8-99.9% of the total canonical correlation energy which is the improvement of 0.2% over the LPNO version in less than 63% of the total LPNO runtime. Similar results were obtained for Iron(II)-porphyrin. When the perturbative triples correction was employed, irrespective of the active space size or system, the obtained energy differences between two spin states were within the chemical accuracy of 1 kcal/mol using the default DLPNO settings.