  We calculate supernova electron capture and beta decay rates for various
pf-shell nuclei using large-scale shell model techniques. We show that the
centroid of the Gamow-Teller strength distribution has been systematically
misplaced in previous rate estimates. Our total electron capture rates are
significantly smaller than currently adopted in core collapse calculations,
while the total beta decay rates change less. Our calculation shows that for
electron-to-baryon ratios Ye=0.42-0.46 beta decay rates are larger than the
competing electron capture rates.
