The electronic and structural properties of the ground and excited states of infinite polydiacetylene chains in acetylenic (PDA) and butatrienic (PBT) structures are studied by a series of ab initio crystalline orbital and linear-combination-of-atomic-orbital periodic density functional theory methods. A complete geometry optimization is performed for PDA and PBT with analytical energy gradient techniques at the Hartree-Fock (HF) and Becke3-Lee-Yang-Parr (B3LYP) levels. The HF/6-31G* and B3LYP/6-31G* reproduce the experimental geometrical parameters of substituted polydiacetylenes with a PDA-like structure. We compute the relative stability and the potential energy curves along the structural transition between PDA and PBT at the HF, B3LYP, and second-order many-body perturbation theory [MBPT(2)] levels. All these calculations predict PDA to be more stable than PBT by 28-87 kJ mol(-1). A minimum corresponding to the PBT-like structure is found at the HF level, but not at the B3LYP or MBPT(2) level. We report the frequencies of all the infrared- and Raman-active vibrational modes of PDA at the HF and B3LYP levels. The frequencies of the carbon backbone stretching modes calculated at the B3LYP/6-31G* level are within 60 cm(-1) of the measured frequencies of resonance Raman bands, when the former values are scaled by a uniform scale factor of 0.96. The ionization potential (IP), electron affinity (EA), and fundamental band gap (E-g) of PDA are calculated at the HF and B3LYP levels and also at the MBPT(2) level employing the quasiparticle formalism. B3LYP/6-31G* provides the most reasonable IP, EA, and E-g, which are within 0.6 eV of the experimental results. Vertical excitation energies to the lowest singlet and triplet excitons of PDA are obtained by configuration interaction singles and by time-dependent density functional theory employing the B3LYP functional. These treatments properly account for the nonvanishing exciton binding energy. While the CIS/6-31G* excessively overestimates the singlet excitation binding energies, B3LYP/6-31G* provides a value (0.3 eV) that is in good agreement with experiment (0.4 eV).