  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.
