Further development of internally contracted multi-reference coupled-cluster theory

• Applicant: Professor Dr. Andreas Köhn
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: from 2011 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 201645791

• Final Report Year: 2022

Final Report Abstract

Multireference coupled-cluster theory aims at providing a reliable standard for accurate quantum chemical calculations of systems with complex electronic structure. In this context, complex electronic structure means that more than one electronic configuration is required to provide a satisfactory zeroth-order description. Examples are polyradicals and transition metal complexes with open d-shells. In this project, we have advanced the development of the internally-contracted multireference coupled-cluster (icMRCC) theory both with respect to the formal background of the theory and by providing more practicable tools for actual computations. We revisited the linked and the unlinked formulation and showed the relation to the coupled-electron pair approximation (CEPA), and investigated the idea of hybrid models, in which computationally challenging terms are replaced by a linearized CEPA-like formulation. We also reviewed a number of technical details, particularly the definition of the non-redundant excitation manifold and the formulation of a perturbative correction for three-electron correlation effects, as required for quantitative predictions. One core part of this project was the efficient reimplementation of icMRCC theory. Progress was hampered by the unforeseen complexity of this undertaking, such that by the end of this funding period, we had to restrict ourselves to a first efficient implementation for the special subcase of complete active space with two-electrons in two orbitals. First tests show a strong improvement of the method’s efficiency relative to our previous implementation, allowing for the treatment of significantly larger systems than before. The new program will become available in a forthcoming release of the Molpro program package.

Publications

• ”Linear and quadratic icMRCC approximations”, Symposium Theoretical Chemistry 2018, Halle (Saale)
  J. A. Black, A. Köhn
  
• “Linear and quadratic contracted multireference coupled-cluster approximations”, J. Chem. Phys. 150, 194107 (2019)
  J. A. Black, A. Köhn
  (See online at https://doi.org/10.1063/1.5095070)
• ”Linear and quadratic and perturbative Internally-contracted multireference coupled-cluster methods”, Molecular Quantum Mechanics 2019, Heidelberg
  J. A. Black, A. Köhn
  
• “Improved and simplified orthogonalisation scheme and connected triples correction within the internally contracted multireference coupled-cluster method”, Mol. Phys. 118, e1743889 (2020)
  A. Köhn, J. A. Black, Y. A. Aoto, M. Hanauer
  (See online at https://doi.org/10.1080/00268976.2020.1743889)
• “A generalized hybrid scheme for multireference methods”, J. Chem. Phys. 155, 204106 (2021)
  A. Waigum, J. A. Black, A. Köhn
  (See online at https://doi.org/10.1063/5.0067511)
• ”A generalized hybrid scheme for multireference methods”, Symposium Theoretical Chemistry 2021, Wurzburg (Online-Event)
  A. Waigum, J. A. Black, A. Köhn
  
• ”Applications of second-order approximate internally contracted multireference coupled-cluster singles and doubles (icMRCC2) theory”, Symposium Theoretical Chemistry 2021, Wurzburg (Online-Event)
  P. Zielinski, J. A. Black, A. Köhn