Density functional calculations have been performed on two types of molecular crystal : (a) crystalline (orthorhombic) polyethylene comprising covalently bonded parallel chains with weak interchain interactions, and (b) a crystalline analog of bisphenol-A polycarbonate with a unit cell containing two molecules with 59 atoms each. The local density approximation for the exchange-correlation energy overestimates the strength of the intermolecular bonds in both, and the Becke-Perdew functional (gradient corrected) gives no intermolecular binding in the former and a very weak bond in the latter. The functional of Perdew, Burke, and Ernzerhof leads to binding in both molecules.