Oscillator strengths in the framework of equation of motion multilevel CC3

We present an efficient implementation of the equation of motion oscillator strengths for the closed-shell multilevel coupled cluster singles and doubles with perturbative triples method (MLCC3) in the electronic structure program eT. The orbital space is split into an active part treated with CC3 and an inactive part computed at the coupled cluster singles and doubles (CCSD) level of theory. Asymptotically, the CC3 contribution scales as $O(n_\text{V} n^3_\text{v} n^3_\text{o})$ floating-point operations (FLOP), where $n_V$ is the total number of virtual orbitals while $n_\text{v}$ and $n_\text{o}$ are the number of active virtual and occupied orbitals, respectively. The CC3 contribution, thus, only scales linearly with the full system size and can become negligible compared to the cost of CCSD. We demonstrate the capabilities of our implementation by calculating the UV-VIS spectrum of azobenzene and a core excited state of betaine 30 with more than 1000 molecular orbitals.