DMRG-Tailored Coupled Cluster Method in the 4c-Relativistic\nDomain: General Implementation and Application to the NUHFI and NUF<sub>3</sub> Molecules

Heavy atom compounds\nrepresent a challenge for computational chemistry\ndue to the need for simultaneous treatment of relativistic and correlation\neffects. Often such systems also exhibit strong correlation, which\nhampers the application of perturbation theory or single-reference\ncoupled cluster (CC) methods. As a viable alternative, we have proposed\n externally correcting the CC method using the density matrix renormalization\ngroup (DMRG) wave functions, yielding the DMRG-tailored CC method.\nIn a previous paper [<i>J. Chem. Phys.</i> <b>2020</b>, <i>152</i>, 174107], we reported a first implementation\nof this method in the relativistic context, which was restricted to\nmolecules with real double group symmetry. In this work, we present\na fully general implementation of the method, covering complex and\nquaternion double groups as well. The 4c-TCC method thus becomes applicable\nto polyatomic molecules, including heavy atoms. For the assessment\nof the method, we performed calculations of the chiral uranium compound\nNUHFI, which was previously studied in the context of the enhancement\nof parity violation effects. In particular, we performed calculations\nof a cut of the potential energy surface of this molecule along the\nstretching of the N–U bond, where the system exhibits strong\nmultireference character. Since there are no experimental data for\nNUHFI, we have performed also an analogous study of the (more symmetric)\nNUF<sub>3</sub> molecule, where the vibrational frequency of the N–U\nbond can be compared with spectroscopic data.