Multireference configuration interaction (MRCI) calculations of the ground state S-2,S-1 and low-lying excited states P-2,P-1, D-2,D-1, and S-2,S-3 of positronium hydride (PsH) were carried out, and their total energies and two-photon annihilation rates were presented. Our MRCI calculations gave more compact and accurate wave functions than recent large scale full CI ones, and the effectiveness of the MRCI method for positron-atom complexes was shown. Two extrapolation methods derived from an analysis of the convergence pattern of the total energies and two-photon annihilation rates with respect to angular momentum of orbitals were attempted in order to estimate full CI limit of those values. As a result, our reliable extrapolation method gave -0.788 642, -0.732 826, -0.724 642, and -0.739 829 a.u. for the total energies of the S-2,S-1, P-2,P-1, D-2,D-1, and S-2,S-3 states of PsH, respectively. For the two-photon annihilation rates of the respective states, 2.1078, 1.5702, 1.0980, and 1.0784 ns(-1) were obtained, respectively. (C) 2003 American Institute of Physics.