A novel method was developed for molecular mechanics calculations and normal mode analysis. In this approach, the number of free parameters is strongly reduced compared with other empirical force fields. and in contrast to them is generally smaller than the number of available wavenumber values. The molecule is subdivided into local units, each of which is constituted by a distinct atom and its nearest neighbors. The vibrational force field is then expressed as the sum over the contributions from all local units, and each local unit's potential function is assumed to depend solely on the atomic positions within the unit. Local units often exhibit high symmetry, because each atom forms bonds which are characteristic of its valencies and hybridization state, and the bonds are therefore arranged in a symmetrical way. This local (pseudo)symmetry imposes group theoretical restrictions that reduce the number of possible interaction parameters. As suggested by ab initio results, the internal force constants of each local unit are transferable to other molecules. It is therefore possible to calculate the internal force constants of each local unit from small molecules and these are then used to calculate the potential of large molecules such as porphyrins. A series of alkanes, ethene, some homo- and heterocyclic aromatic compounds and porphyrins were analyzed. The results for the normal mode wavenumbers and their eigenvectors are comparable to those reported in the literature and to results from DFT calculations [B3-LYP/6–31G(d)]. The force constants were close to those obtained from ab initio calculations using local symmetry coordinates for ethene, ethane and propane. Moreover, the above procedure reproduces very well the vibrational wavenumbers and mode compositions of aromatic compounds and porphyrins, as shown by comparison with DFT calculations. In contrast to general valence force field calculations, the number of free parameters is reduced by 40–80%. Copyright © 1999 John Wiley & Sons, Ltd.