Modification of optical phonon spectra in contacting nonpolar nanoparticles compared to single particles
is studied. Optical phonons in dielectric and semiconducting particles obey the Euclidean metric Klein-FockGordon equation with Dirichlet boundary conditions. This equation is supposed to be solved numerically for
manifolds of cojoined spheres. It is proposed to replace this problem with the simpler-to-solve coupled-oscillator
model (COM), where an oscillator is attributed to each phonon mode of a particle and the particle overlap
leads to the appearance of additional couplings for these oscillators with the magnitude proportional to the
overlap volume. For not too big overlaps, this model describes solutions of the original eigenvalue problem
with a quantitative level of accuracy. In particular, it works beyond isotropic s modes in dimers, which has been
demonstrated for p modes in dimers and for tetramers. It is proposed to apply the COM for the description
of recently manufactured dimer nanoparticles and quantum dots. The obtained results are in agreement with
the dynamical matrix method for optical phonons in nanodiamonds. The dynamical matrix method is also used
to demonstrate that the van der Waals contacts between faceted particles lead to very small modifications of
the optical phonon spectra, which therefore could be neglected when discussing the propagation of vibrational
excitations via a nanopowder. The possibility to distinguish between dimerized and size-distributed single
particles from their Raman spectra is also considered. The proposed COM paves a way towards the description
of propagation of vibrational modes in the ensembles of particles in contact including tight agglomerates,
nanocrystal solids, and porous media.