Rank-Reduced Equation-of-Motion Coupled Cluster Formalism with Full Inclusion of Triple Excitations

In this work, we describe the rank-reduced variant of the equation-of-motion coupled cluster theory with complete inclusion of single, double, and triple excitations. The advantage of the proposed formalism, in comparison with the canonical theory, stems from the application of Tucker decomposition format to the ground- and excited-states triply excited amplitude tensors. By exploiting the linear scaling of the dimension of the decomposed amplitudes with respect to the system size N, one can reduce the computational cost of the method to the level of N⁶ and storage requirements to N⁴. While, in practice, the proposed rank-reduced formalism introduces an error, we show that it is several times smaller than the inherent error of the parent theory with the proposed default settings for a wide range of problems. A higher level of accuracy can be achieved by increasing the value of a single parameter present in this formulation, recovering the canonical method in an appropriate limit. We illustrate the accuracy and performance of the proposed method by calculations for a group of molecules with excited states of different character─from dominated by single excitations with respect to the reference determinant to states with moderate and large contributions of double and higher excitations. We report calculations of potential energy curves and related spectroscopic parameters for the first four singlet excited states of magnesium dimer, as well as the potential energy curve for the excited state of charge-transfer character in the NH₃-F₂ complex as a function of intermolecular separation.