  The theory developed in our previous papers is applied in this paper to
investigate the dependence of slowing down of dynamics of glass-forming liquids
on the attractive and repulsive parts of intermolecular interactions. Through
an extensive comparison of the behavior of a Lennard-Jones glass-forming liquid
and that of its WCA reduction to a model with truncated pair potential without
attractive tail, we demonstrate why the two systems exhibit very different
dynamics despite having nearly identical pair correlation functions. In
particular, we show that local structures characterized by number of mobile and
immobile particles around a central particle markedly differ in the two systems
at densities and temperatures where their dynamics show large difference and
nearly identical where dynamics nearly overlap. We also show how the parameter
{\psi}(T ) that measures the role of fluctuations embedded in the system on
size of the cooperatively reorganizing cluster (CRC) and the crossover
temperature T a depend on the intermolecular interactions. These parameters
stemming from the intermolecular interactions characterize the temperature and
density dependence of structural relaxation time {\tau} {\alpha}. The
quantitative and qualitative agreements found with simulation results for the
two systems suggest that our theory brings out the underlying features that
determine dynamics of glass-forming liquids.
