  First principle calculations of the Raman intensities of a (BEDT-TTF)^+_2
centrosymmetric dimer lead to a full reconsideration of the assignment of the
three C=C stretching phonons in \k{appa}-phase BEDT-TTF salts. In contrast with
previous interpretation, it is found that the presence of the out-of-phase
coupling of the antisymmetric external C=C stretching mode has also to be taken
into account. This mode, infrared-active for a single BEDT-TTF molecule,
implies a strong intra-molecular charge oscillation along BEDT-TTF long
molecular axis. In the consequent reassignment of the C=C spectral region, a
very broad band appearing in the cross polarized Raman spectra is interpreted
as due to inter -dimer electron-molecular vibration (e-mv) coupling, to be
contrasted with the well known intra-dimer e-mv coupling that induces very
strong infrared absorptions. Analysis of the data in term of a properly
developed e-mv scheme yields an evaluation of the Hubbard parameters relevant
to the so-called effective dimer model often used to interpret the physical
properties of \k{appa}-BEDT-TTF salts. Finally the Raman spectra of an
asymmetric (BEDT-TTF)^{+0.6} (BEDT-TTF)^{+0.4} dimer have been calculated in
order to interpret the Raman spectra of the intriguing charge-ordered (CO),
ferroelectric phase of \k{appa}-(BEDT-TTF)_2 Hg(SCN)_2 Cl. It turns out that in
the asymmetric dimer the strong intra-dimer the e-mv induced infrared C=C
stretching absorption should appear with huge intensity also in Raman. The
absence of such band in the spectra of \k{appa}-(BEDT-TTF)_2 Hg(SCN)_2 Cl
ferroelectric phase gives clear alternative indications about the possible CO
pattern in such a phase.
