The lowest 42 electronic states of diatomic gas phase gold silicide are calculated by complete active space self-consistent field and second-order multiconfigurational perturbation theory. The scalar relativistic effect is considered by the third-order Douglas-Kroll method and the spin-orbit coupling is treated perturbatively after the spin-free calculations. The ground state of AuSi is confirmed to be the (2)Pi(1/2) state, and the spin-orbit splitting of (2)Pi(1/2) to (2)Pi(3/2) is estimated as 1527 cm(-1). From the careful analysis of the potential energy curves and the spectroscopic constants, the experimentally observed A and D excited states are assigned to Omega=1/2(II) or Omega=1/2(III) and Omega=1/2(V) or Omega=1/2(VI), respectively. (C) 2002 American Institute of Physics.