Optimized molecular structures and k=0 (Brillouin zone center) vibrational frequencies are obtained for trans- and cis-transoid polyacetylene and for anti- and syn-transoid polymethineimine by the density functional crystal orbital method with the Becke3-Lee-Yang-Parr functional. The analytical energy gradient scheme is implemented in the density functional crystal orbital method, and the force constants of the infinite polymers are evaluated by numerical differentiation of the analytical energy gradients. For the trans- and cis-transoid isomers of polyacetylene, the vibrational frequencies calculated and then uniformly scaled by a single scale factor are in reasonable agreement with the observed frequencies. For polymethineimine, it is found that the calculated frequencies of the anti-transoid isomer completely disagree with the observed frequencies. In contrast, the calculated frequencies of syn-transoid polymethineimine are in agreement with the observed, although there are only three observed frequencies available. The total energy of syn-transoid polymethineimine is found to be lower than that of the anti-transoid isomer by 15.6 kJ mol(-1). (C) 1997 American Institute of Physics. [S0021-9606(97)02547-6].