  Vibrational spectra can be computed without storing full-dimensional vectors
by using low-rank sum-of-products (SOP) basis functions. We introduce symmetry
constraints in the SOP basis functions to make it possible to separately
calculate states in different symmetry subgroups. This is done using a power
method to compute eigenvalues and an alternating least squares method to
optimize basis functions. Owing to the fact that the power method favours the
convergence of the lowest states, one must be careful not to exclude basis
functions of some symmetries. Exploiting symmetry facilitates making
assignments and improves the accuracy. The method is applied to the
acetonitrile molecule.
