  Ultracold atoms confined in a dipole trap are submitted to a potential whose
depth is proportional to the real part of their dynamic dipole polarizability.
The atoms also experience photon scattering whose rate is proportional to the
imaginary part of their dynamic dipole polarizability. In this article we
calculate the complex dynamic dipole polarizability of ground-state erbium, a
rare-earth atom that was recently Bose-condensed. The polarizability is
calculated with the sum-over-state formula inherent to second-order
perturbation theory. The summation is performed on transition energies and
transition dipole moments from ground-state erbium, which are computed using
the Racah-Slater least-square fitting procedure provided by the Cowan codes.
This allows us to predict 9 unobserved odd-parity energy levels of total
angular momentum J=5, 6 and 7, in the range 25000-31000 cm-1 above the ground
state. Regarding the trapping potential, we find that ground-state erbium
essentially behaves like a spherically-symmetric atom, in spite of its large
electronic angular momentum. We also find a mostly isotropic van der Waals
interaction between two ground-state erbium atoms, characterized by a
coefficient C_6^{iso}=1760 a.u.. On the contrary, the photon-scattering rate
shows a pronounced anisotropy, since it strongly depends on the polarization of
the trapping light.
