Ab initio methods targeting strong electron- and nuclear-correlation in spectroscopy

The density matrix renormalization group (DMRG) has emerged as an important alternative to multi-configurational self-consistent-field calculations in molecular spectroscopy, replacing standard complete active space approaches for large orbital spaces. We implemented a spin-adapted matrix-product-state (MPS) and -operator formulation of the DMRG, which served as a flexible and efficient basis for the development of an MPS state-interaction approach for optical spectroscopy and of a short-time resonance Raman spectroscopy model. We extended our approach to vibrational spectroscopy and I will discuss how the DMRG can be exploited to optimize vibrational wave functions expressed as matrix product states.