  A new Monte Carlo algorithm for calculating polaron effective mass is
proposed. It is based on the path-integral representation of a partial
partition function with fixed total quasi-momentum. Phonon degrees of freedom
are integrated out analytically resulting in a single-electron system with
retarded self-interaction and open boundary conditions in imaginary time. The
effective mass is inversely proportional to the covariance of total energy
calculated on an electron trajectory and the square distance between ends of
the trajectory. The method has no limitations on values of model parameters and
on the size and dimensionality of the system although large statistics is
required for stable numerical results. The method is tested on the
one-dimensional Holstein model for which simulation results are presented.
