6th International Conference on Molecular Electronic Structure, (MES), Monastir, Tunisia, 2 - 06 September 2022, pp.15
High-accuracy computational chemistry provides a powerful tool that can augment the body of experimental data, particularly in case of fleeting intermediates or hazardous compounds. A number of composite schemes have been proposed in the recent decades that aim at the accurate determination of thermochemical properties with sub-chemical accuracy [1-5]. However, most of these protocols defined in this manner are based on single reference methods.
In this study, we evaluate the option of integrating the internally-contracted multireference coupled-cluster (icMRCC) method [6,7] into a high-accuracy thermochemistry protocol. Unlike multireference configuration interaction (MRCI), icMRCC is a size-consistent method and promises uniform accuracy independent of the systems size. The icMRCC approach is a straightforward generalization of the standard coupled-cluster method which bears the option of integrating single-reference and multireference schemes.
In a first step, we evaluate the accuracy of the icMRCC approach, addressing in particular the question how the accuracy of icMRCCSD and icMRCCSD(T) compares to the usual coupled-cluster hierarchy for different cases. We address the question of how certain approximations for the triples approximation [6,7] impact the overall accuracy of the method, in particular if large basis sets are involved. We also investigate the question of choosing active spaces and extrapolating to the complete basis set limit for multireference schemes. Comparison is made to the HEAT protocol for obtaining accurate atomization and reaction energies. We also compare to experimental results from the Active Thermochemical Tables (ATcT). 
Acknowledgements: This work has been financially supported by the Scientific and Technological Research Council of Turkey-TÜBİTAK, Grant no: TÜBİTAK-BIDEB-2219 International Postdoctoral Research Fellowship Program (2019-1).The authors acknowledge support by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/575-1 FUGG (JUSTUS 2 cluster).
 L. A. Curtiss, K. Raghavachan, P. C. Redfern, V. Rassolov, J. A. Pople, J. Chem. Phys. 109, 7764-7776, 1998.
 A. Tajti, P. G. Szalay, A. G. Csaszar, M. Kallay, J. Gauss, E. F. Valeev, B. A. Flowers, J.Vazquez, J. F. Stanton, J. Chem. Phys. 121, 11599-11613 (2004).
 P. L. Fast, N. E. Schultz, D. G. Truhlar, J. Phys. Chem. A, 105, 4143-4149, 2001.
 D. Feller, K. A. Peterson, D. A. A. Dixon, J. Chem. Phys., 129, 204105/1-32, 2008.
 A. Karton, E. Rabinovich, J. M. L. Martin, B. Ruscic, J. Chem. Phys., 125, 144108/1-17, 2006.
 M. Hanauer, A. Köhn, J. Chem. Phys. 136, 204107 (2004).
 A. Köhn, J. A. Black, Y. A. Aoto, M. Hanauer, Mol. Phys. 118, e1743889 (2020).
 B. Ruscic, R. E. Pinzon, M. L. Morton, G. von Laszewski, S. Bittner, S. G. Nijsure, K. A. Amin, M. Minkoff, and A. F. Wagner, J. Phys. Chem. A 108, 9979, 2004.