  We extend the Feynman variational method applied to the parabolic-band
Fr\"ohlich (continuum) large polaron~\cite{Feynman1955} to a Holstein (lattice)
small polaron, with a parabolic-band. This new theory shows a discrete
localisation as a function of coupling strength. Having build the theory with
the same quasi-particle Lagrangian as the 1955 work, we can directly use the
FHIP~\cite{Feynman1962} response theory to calculate DC mobility and complex
conductivity. We show that we can take matrix elements from electronic
structure calculations on real materials, by modelling charge-carrier mobility
in crystalline Rubrene. Good agreement is found to measurement, with a
predicted mobility of $\mu = 47.72$~\si{cm^2 V^{-1} s^{-1}} at $300$~\si{K}.
