  Organic charge transfer salts exhibit thermal expansion anomalies similar to
those found in other strongly correlated electron systems. The thermal
expansion can be anisotropic and have a non-monotonic temperature dependence.
We show how these anomalies can arise from electronic effects and be
significantly enhanced, particularly at temperatures below 100 K, by strong
electronic correlations. For the relevant Hubbard model the thermal expansion
is related to the dependence of the entropy on the parameters ($t$, $t'$, and
$U$) in the Hamiltonian or the temperature dependence of bond orders and double
occupancy. The latter are calculated on finite lattices with the Finite
Temperature Lanczos Method. Although many features seen in experimental data,
in both the metallic and Mott insulating phase, are described qualitatively,
the calculated magnitude of the thermal expansion is smaller than that observed
experimentally.
