  A variant of White's density matrix renormalisation group scheme which is
designed to compute low-lying energies of one-dimensional quantum lattice
models with a large number of degrees of freedom per site is described. The
method is tested on two exactly solvable models---the spin-1/2
antiferromagnetic Heisenberg chain and a dimerised XY spin chain. To illustrate
the potential of the method, it is applied to a model of spins interacting with
quantum phonons. It is shown that the method accurately resolves a number of
energy gaps on periodic rings which are sufficiently large to afford an
accurate investigation of critical properties via the use of finite-size
scaling theory.
