Density functional theory B3LYP and ab initio MP2 calculations were carried out to study the structures and vibrational spectra of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the related compounds, 1,3-dithiole-2-thione (DTT), tetrathiafulvalene (TTF), and 4,5-ethylenedithio-1,3-dithiole-2-thione (EDT-DTT). It is found that B3LYP functional overestimates C-S bond lengths significantly and underestimates frequencies of modes involving C-S stretching accordingly. The errors in B3LYP force fields are shown to be satisfactorily corrected by scaled quantum mechanical force field procedure. After applying the scale factors derived from DTT, the scaled B3LYP force fields reproduce the observed frequencies of TTF, EDT-DTT, and BEDT-TTF satisfactorily, with a mean deviation between the calculated and observed frequencies of less than 10 cm-1. On the basis of agreement between the calculated and observed frequencies, isotope shifts, as well as IR and Raman intensities, assignments of the fundamental vibrational modes of these molecules are given in terms of the true molecular symmetries of the equilibrium structures. This study shows that the scaled density functional force field procedure is a powerful approach for understanding the spectral features of large and low symmetry molecules.