  The coefficients of different mass formulae derived from the liquid drop
model and including or not the curvature energy, the diffuseness correction to
the Coulomb energy, the charge exchange correction term, different forms of the
Wigner term and different powers of the relative neutron excess $I=(N-Z)/A$
have been determined by a least square fitting procedure to 2027 experimental
atomic masses. The Coulomb diffuseness correction $Z^2/A$ term or the charge
exchange correction $Z^{4/3}/A^{1/3}$ term plays the main role to improve the
accuracy of the mass formula. The Wigner term and the curvature energy can also
be used separately for the same purpose. The introduction of an $|I|$
dependence in the surface and volume energies improves slightly the efficiency
of the expansion and is more effective than an $I^4$ dependence. Different
expressions reproducing the experimental nuclear charge radius are provided.
The different fits lead to a surface energy coefficient of around 17-18 MeV and
a relative equivalent rms charge radius r$_0$ of 1.22-1.23 fm.
