  It has long been suggested that shocks might play an important role in
altering the form of the interstellar medium (ISM). Shocks enhance gas density
and sufficiently dense regions may become self gravitating. Potential star
forming clouds within larger molecular clouds, move randomly at supersonic
speeds.
  Depending on the precollision velocity, colliding molecular clouds produce a
slab that is either shock compressed or pressure confined. In a sequel of two
papers (I & II), we simulate molecular cloud collision and investigate the
dynamical evolution of such slabs. Shocked slabs are susceptible to
hydrodynamic instabilities and in the present paper (I) we study the effect of
strong shear between slab layers on the dynamic evolution of a shock compressed
gas slab. Both, head-on and off-centre cloud collisions have been examined in
this work. We include self gravity in all our simulations.
  Simulations presented here, are performed using the smoothed particle
hydrodynamics (SPH) numerical scheme. Individual, pre-collision clouds are
modelled as pressure confined Bonnor-Ebert spheres. However, in the interest of
brevity the thermodynamic details of the problem are simplified and the gas
temperature is simply evolved by a barytropic equation of state. Obviously, the
gas, to some extent suffers from thermal inertial effects. However, we note
that the dynamical timescale is much smaller than the local sound crossing time
so that such effects should have minimum influence.
