  On any Calabi-Yau manifold X one can define a certain sheaf of chiral N=2
superconformal field theories, known as the chiral de Rham complex of X. It
depends only on the complex structure of X, and its local structure is
described by a simple free field theory. We show that the cohomology of this
sheaf can be identified with the infinite-volume limit of the half-twisted
sigma-model defined by E. Witten more than a decade ago. We also show that the
correlators of the half-twisted model are independent of the Kahler moduli to
all orders in worldsheet perturbation theory, and that the relation to the
chiral de Rham complex can be violated only by worldsheet instantons.
