  The incompatibility of partial d occupation on the perovskite B-site with the
standard charge transfer mechanism for ferroelectricity has been a central
paradigm in multiferroics research. Nevertheless, it was recently shown by
density functional theory calculations that CaMnO_3 exhibits a polar
instability that even dominates over the octahedral tilting for slightly
enlarged unit cell volume. Here, we present similar calculations for LaCrO_3,
which has the same d^3 B-site electron configuration as CaMnO_3. We find that
LaCrO_3 exhibits a very similar, albeit much weaker, polar instability as
CaMnO_3. In addition, while the Born effective charge (BEC) of the Mn^{4+}
cation in CaMnO_3 is highly anomalous, the BEC of Cr^{3+} in LaCrO_3 is only
slightly enhanced. By decomposing the BECs into contributions of individual
Wannier functions we show that the ferroelectric instabilities in both systems
can be understood in terms of charge transfer between TM d and O p states,
analogously to the standard d^0 perovskite ferroelectrics.
