The surface phase transition between GaAs(001)-c(4 x 4) and -(2 x 4)beta 2 surfaces is systematically investigated by using our ab initio-based approach. The phase diagram calculations for the c(4 x 4) surfaces as functions of temperature and As pressure reveal that three kinds of c(4 x 4) surfaces consisting of Ga-As dimers and/or Ga-Ga dimers become stable near the phase transition temperature. The electron counting Monte Carlo simulation and ab initio calculations are also performed to investigate the structural change of the c(4 x 4) surface after predepositing a 0.5 monolayer of Ga on the three kinds of c(4 x 4) surfaces. The calculated results suggest that the c(4 x 4) surfaces consisting of three Ga-As dimers or one Ga-Ga dimer and two Ga-As dimers in the (4 x 4) surface unit cell possibly change their structures to (2 x 4)beta 2 structures with Ga-As surface dimers. The conventional (2 x 4)beta 2 surface consisting of As dimers finally appears due to destabilization of Ga-As dimers at high temperature and high pressure. (c) 2004 Elsevier B.V. All rights reserved.