Reference spaces for multireference coupled-cluster theory: the challenge of the CoH molecule

• Published in: Theoretical chemistry accounts 2020-04, Vol.139 (4), Article 71
• Main Authors: de Moraes, Matheus Morato F., Aoto, Yuri Alexandre
• Format: Article
• Language: English
• Subjects: Atomic/Molecular Structure and Spectra; Chemistry; Chemistry and Materials Science; Clusters; Configuration interaction; Coupling (molecular); Electron states; Inorganic Chemistry; Mathematical analysis; Metal compounds; Orbitals; Organic Chemistry; Physical Chemistry; Potential energy; Prof. Fernando R. Ornellas Festschrift; Regular Article; Theoretical and Computational Chemistry; Transition metal compounds
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-020-2584-1

In multireference calculations on transition metal compounds, large active spaces including a second set of d orbitals are often used. However, with the increase in the system size and the complexity of the method, such calculations quickly become impractical. In this work, we looked for an inner valence active space for atomic cobalt and its hydride that correctly describes the transition energies among states with distinct configurations. Potential energy curves for the CoH molecule were obtained using a fixed reference space, where the orbitals are optimised only for the isolated atoms, avoiding undesirable sudden variations of the active orbitals. The calculations made use of the multireference configuration interaction (MRCI) and coupled-cluster (MRCC) theories. The accuracy of these calculations is carefully analysed, and we show that MRCC results are accurate, although not all electronic states are quantitatively described at the MRCI level. We conclude that the use of the fixed reference space is a viable approach to reduce the computational cost when large active spaces are prohibitive. Finally, a careful comparison of the present results with experimental values is carried out.