We present results of first-principles plane wave density functional studies of the polycyclic hydrocarbons pentaprismane and hypostrophene. which demonstrate, for the first time, that a plane wave basis can be successfully deployed with such strained molecules. Calculated structural parameters are found to be in good agreement with previous quantum chemical calculations and X-ray diffraction data (where available). A series of calculations for related C10H10 isomers, in which the five-fold rings of pentaprismane and hypostrophene are cleaved progressively further apart, are also reported. The way is now open for the calculation of the properties of these complex molecules on surfaces. (C) 2000 Elsevier Science B.V. All rights reserved.