  Liquid hydrocarbons are often modeled with fixed, symmetric, atom-centered
charge distributions and Lennard-Jones interaction potentials that reproduce
many properties of the bulk liquid. While useful for a wide variety of
applications, such models cannot capture dielectric effects important in
solvation, self-assembly, and reactivity. The dielectric constants of
hydrocarbons, such as methane and ethane, physically arise from electronic
polarization fluctuations induced by the fluctuating liquid environment. In
this work, we present non-polarizable, fixed-charge models of methane and
ethane that break the charge symmetry of the molecule to create fixed molecular
dipoles, the fluctuations of which reproduce the experimental dielectric
constant. These models can be considered a mean-field-like approximation that
can be used to include dielectric effects in large-scale molecular simulations
of polar and charged molecules in liquid methane and ethane. We further
demonstrate that solvation of model ionic solutes and a water molecule in these
fixed-dipole models improve upon dipole-free models.
