Here, we present two-dimensional numerical calculations of the head-on and off-centre binary collision dynamics of equal-sized, van der Waals liquid drops using the method of smoothed particle hydrodynamics (SPH). Because of the planar geometry used to represent the drops, the simulations apply to the collision of two infinitely long cylinders. The outcome of coalescence, in which two drops combine permanently, is studied only for low energy impact collisions with Reynolds (Re) and Weber (We) numbers in the ranges 23 <= Re <= 68 and 1 <= We <= 10, respectively. In particular, the effects of varying both the impact velocity and the impact parameter on the outcome of permanent coalescence are investigated. (c) 2005 Elsevier Ltd. All rights reserved.