  In this paper we present a critical discussion of the algorithms commonly
used in N-body simulations of Galaxy Formation to deal with the energy equation
governing heating and cooling, to model star formation and the star formation
rate, and to account for energy feed-back from stars. First, we propose our
technique for solving the energy equation in presence of heating and cooling,
which includes some difference with respect to the standard semi-implicit
technique. Second, we examine the current criteria for the onset of the star
formation activity. We suggest a new approach, in which star formaiton is let
depend on the total mass density - baryonic (gas and stars) and dark matter -
of the system and on the metal-dependent cooling efficiency. Third, we check
and discuss the separate effects of energy (and mass) feed-back from several
sources - namely supernovae, stellar winds from massive stars, and UV flux from
the same objects. All the simulations are performed in the framework of the
formation and evolution of a disk galaxy. We show that the inclusion of these
physical phenomena has a signigicant impact on the evolution of the galaxy
model.
