Ab initio total energy calculations at the DFT-GGA level for PIPD are reported. Both the monoclinic crystal with a bi-directional hydrogen-bond network and the triclinic crystal with a sheet-like network are studied. It is concluded that the latter is the more plausible microstructure for the fibre based on the following: (i) After optimisation of the lattice parameters and atomic positions it has a lower energy. (ii) The calculated internal shear modulus agrees better with experiment. (iii) The minimal shear stiffness constant, which is interpreted as the upper limit on the compressive strength, compares favourably with the experimental compressive strength. The hydrogen bonding network plays a crucial-but indirect-role in explaining the high compressive strength. It replaces the weak components of the lateral bonding, such as present in many high performance polymer materials with low compressive strength, e.g. PBO and PBZT, with much stronger hydrogen bonds. This makes that in PIPD the relatively strong pi-pi interaction has the weakest resistance against shear. (c) 2005 Elsevier Ltd. All rights reserved.