  A simple way to investigate theoretically the Raman spectra (RS) of nonpolar
nanoparticles is proposed. For this aim we substitute the original lattice
optical phonon eigenproblem by the continuous Klein-Fock-Gordon-like equation
with Dirichlet boundary conditions. This approach provides the basis for the
continuous description of optical phonons in the same manner how the elasticity
theory describes the longwavelength acoustic phonons. Together with continuous
reformulation of the bond polarization model it allows to calculate the RS of
nanoparticles without referring to their atomistic structure. It ensures the
powerful tool for interpreting the experimental data, studying the effects of
particle shape and their size distribution. We successfully fit recent
experimental data on very small diamond and silicon particles, for which the
commonly used phonon confinement model fails. The predictions of our theory are
compared with recent results obtained within the dynamical matrix method - bond
polarization model (DMM-BPM) approach and an excellent agreement between them
is found. The advantages of the present theory are its simplicity and the
rapidity of calculations. We analyze how the RS are affected by the
nanoparticle faceting and propose a simple power law for Raman peak position
dependence on the facets number. The method of powder RS calculations is
formulated and the limitations on the accuracy of our analysis are discussed.
