  The applicability of the three steps systematic parametrization procedure
(3SSPP) to develop a force field for primary amines was evaluated in the
present work. Previous simulations of primary amines show that current force
fields (FF) can underestimate some experimental values under room conditions.
Therefore, we propose a new set of parameters, for an united atom (UA) model,
that can be used for short and long amines which predict correctly
thermodynamic and dynamical properties. Following the 3SSPP methodology, the
partial charges are chosen to match the experimental dielectric constant
whereas the Lennard-Jones (LJ) parameters, $\epsilon$ and $\sigma$, are fitted
to reproduce the surface tension at the vapor-liquid interface and the liquid
density, respectively. Simulations were initially conducted for the propylamine
molecule by introducing three different types of carbon atoms, C$_\alpha$ and
C$_\beta$, with electric charges, and C$_n$, without charge. Then, modifying
the charges of the carbons and using the transferable LJ parameters, the new
set of constants for long amines were found. The results show good agreement
for the experimental dielectric constant and mass density with a percentage
error less than 1%, whereas for the surface tension the error is up to 4%. For
the short amines, methylamine and ethylamine, the new charges were obtained
from a fitting function calculated from the long amines results. For these
molecules, the values of the dielectric constant and the surface tension
present errors of the order of 10% with the experimental data. Miscibility of
the amines was also tested with the new parameters and the results show
reasonable agreement with experiments.

Soft Condensed Matter;Chemical Physics