  A class of Fourier Integral Operators which converge to the unitary group of
the Schroedinger equation in semiclassical limit $\eps\to 0$ is constructed.
The convergence is in the uniform operator norm and allows for an error bound
of order $O(\eps^{1-\rho})$ for Ehrenfest timescales, where $\rho$ can be made
arbitrary small. For the shorter times of order O(1), the error can be improved
to arbitrary order in $\eps$. In the chemical literature the approximation is
known as the Herman-Kluk propagator.
