We have investigated the effect of the initial condition on the spreading exponents of the one-dimensional pair contact process (PCP) and threshold transfer process. The nonorder field was found to exhibit critical fluctuations, relaxing to its natural value with the same power law as the order parameter field. We argue that this slow relaxation, which was not taken into account in earlier studies of these models, is responsible for the continuously changing survival probability exponent. High-precision numerical simulations show evidence of a (slight) dependence of the location of the transition point on the initial concentration, in the case of PCP. The damage spreading point and the spreading exponents coincide with those of the ordinary critical point in both cases.

• Received 6 July 1998

DOI:https://doi.org/10.1103/PhysRevE.58.7020