  Energies for the lowest 56 levels, belonging to the 3s$^2$3p, 3s3p$^2$,
3p$^3$, 3s$^2$3d, 3s3p3d, 3s$^2$4$\ell$ and 3s$^2$5$\ell$ configurations of Si
II, are calculated using the {\sc grasp} (General-purpose Relativistic Atomic
Structure Package) code. Analogous calculations have also been performed (for
up to 175 levels) using the Flexible Atomic Code ({\sc fac}). Furthermore,
radiative rates are calculated for all E1, E2, M1 and M2 transitions. Extensive
comparisons are made with available theoretical and experimental energy levels,
and the accuracy of the present results is assessed to be better than 0.1 Ryd.
Similarly, the accuracy for radiative rates (and subsequently lifetimes) is
estimated to be better than 20% for most of the (strong) transitions. Electron
impact excitation collision strengths are also calculated, with the Dirac
Atomic R-matrix Code ({\sc darc}), over a wide energy range up to 13 Ryd.
Finally, to determine effective collision strengths, resonances are resolved in
a fine energy mesh in the thresholds region. These collision strengths are
averaged over a Maxwellian velocity distribution and results listed over a wide
range of temperatures, up to 10$^{5.5}$ K. Our data are compared with earlier
$R$-matrix calculations and differences noted, up to a factor of two, for
several transitions. Although scope remains for improvement, the accuracy for
our results of collision strengths and effective collision strengths is
assessed to be about 20% for a majority of transitions.

Instrumentation and Methods for Astrophysics;Atomic Physics