Let's assume I have already relaxed a molecular structure for a non-trivial molecule and obtained a geometry that is close to the one resulting from X-ray crystallography. This can be achieved in a number of ways, e.g. trivially (relaxation in vacuum) if the molecule is not feeling a significant distortion from its environment, or with ad-hoc tricks such as surrounding it with a frozen local environment. In any case, I have access to a set of vibrational normal modes, that, in a first approximation and for my purposes, may resemble the actual vibrations of the molecules in the crystal. This can be done with a variety of packages, let's say we are using Gaussian.
Independently, we have information on the phonon spectrum. Again, this can be done with a variety of packages, let's say we are using Quantum Espresso.
The question is: is there a good procedure to map (or, at least, to relate) the molecular normal modes with the lattice phonons? In particular I'm interested in complicated (C1 symmetry) molecules, so any way that does not require relying on symmetry arguments would be preferred.
To put the question into perspective: the goal would be to facilitate the molecular design of systems with improved properties. Designing and obtaining molecules is hard enough as it is, but designing specific crystal packings is adding a layer of complexity, hence the utility of projecting (most of) the key behavior into the single molecule level, whenever it is possible.