  In this work, a new force field is presented for the ionic liquid
1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide, [Bmim][Nf$_2$T].
As a part of the \epsilon force field, the acronym IL/\epsilon is used to refer
to this ionic liquid. This new force field reproduces the dielectric constant,
the density, and the entalphy of vaporization, with an error of less than 3%,
being posible to generate new force fields for ionic liquids, based on the
flexibility of the molecule. In addition, a study of the [Bmim][Nf$_2$T]-H$_2$O
mixture is performed from pure IL to pure water, passing through various
concentrations.
