  The spherical infall model first developed by Lema\^{i}tre and Tolman was
modified in order to include the effects of a dark energy term. The resulting
velocity-distance relation was evaluated numerically. This equation, when
fitted to actual data, permits the simultaneous evaluation of the central mass
and of the Hubble parameter. Application of this relation to the Local Group,
when the dark energy is modeled by a cosmological constant, yields a total mass
for the M31-Milky Way pair of (2.5 +/- 0.7) x 10^12 M\_sun, a Hubble parameter
H\_0 = 74 +/- 4 km s^-1 Mpc^-1 and a 1-D velocity dispersion for the flow of
about 39 km s^-1. The zero-velocity and the marginally bound surfaces of the
Local Group are at about 1.0 and 2.3 Mpc respectively from the center of mass.
A similar analysis for the Virgo cluster yields a mass of (1.10 +/- 0.12) x
10^15 M\_sun and H\_0 = 65 +/- 9 km s^-1 Mpc^-1. The zero-velocity is located
at a distance of 8.6 +/- 0.8 Mpc from the center of the cluster. The predicted
peculiar velocity of the Local Group towards Virgo is about 190 kms^-1, in
agreement with other estimates. Slightly lower masses are derived if the dark
energy is represented by a fluid with an equation of state P = w\epsilon with w
= -2/3.
