A detailed investigation of the geometric structure, force field, electro-optical parameters, relative IR intensities and harmonic vibrational wavenumbers of free 4-aminopyrimidine molecule (4APM) in the electronically ground state has been carried out by using both the DFT-B3LYP (with 6-31 + +G(d,p) double and 6-311 + +G(d,p) triple basis sets) and force field refinement calculation methods. The vibrational wavenumbers calculated with DFT method were scaled by using two different methods: (1) scaling with dual scaling factors, (2) deriving the scaling factors from the graph of observed versus calculated wavenumbers. In the case of force field refinement method, the force constants of the pyrimidine were slightly refined so as to fit the calculated wavenumbers to the experimental ones. In order to define the contributions of the internal coordinates of the molecule on its each normal vibrational mode, P.E.D. calculations were performed. In wavenumber and PED calculations, both methods have yield results in agreement with the experimental assignment and also with each other, particularly for ring vibrations. The relative IR intensities calculated by the force field refinement method are considerably in good agreement with experimental ones, however, the results of the IR intensities, obtained from the DFT method are found to be significantly different from the experimental values. (c) 2004 Elsevier B.V. All rights reserved.