  We have developed a method for calculating the two-point correlation function
of nonlinearly evolved mass and collapsed halos in the Press-Schechter
formalism. The nonlinear gravitational interaction is treated as the sum of
various individual spherical top-hat clustering. Because no collapsed halo of
mass M can exist in initial regions (or top-hat spheres) of mass less than M,
the bias that massive halos have stronger correlation than the background mass
can be naturally introduced. We apply this method to derive constraints on
popular dark-matter models from the spatial number density and the correlation
function of C IV absorption systems in QSO spectra. Considering C IV systems
should behosted by collapsed halos, one can obtain an upper limit to the
threshold mass of the collapsed halos by requiring their number density to be
larger than that of observed C IV systems. On the other hand, in order to
explain the observed clustering of C IV systems, a lower limit to the threshold
mass will be set for the hosting halos. The cold dark matter model, the Lamda
CDM model and the cold-plus-hot dark matter model are tested using the C IV
observations.
