Flexible DMRG-based framework for anharmonic vibrational calculations

We present a novel formulation of the vibrational density matrix renormalization group (vDMRG) algorithm tailored to strongly anharmonic molecules described by general high-dimensional model representations of potential energy surfaces. For this purpose, we extend the vDMRG framework to support vibrational Hamiltonians expressed in the so-called $n$-mode second-quantization formalism. The resulting $n$-mode vDMRG method offers full flexibility with respect to both the functional form of the PES and the choice of the single-particle basis set. We leverage this framework to apply, for the first time, vDMRG based on an anharmonic modal basis set optimized with the vibrational self-consistent field algorithm on an on-the-fly constructed PES. We also extend the $n$-mode vDMRG framework to include excited-state targeting algorithms in order to efficiently calculate anharmonic transition frequencies. We demonstrate the capabilities of our novel $n$-mode vDMRG framework for methyloxirane, a challenging molecule with 24 coupled vibrational modes.