SCF calculations have been carried out for the ground 1Σ g+(0g+) state of Au2 using a variety of ab initio effective core potentials (EP). The effective core potentials studied both include a two-component relativistic EP (REP) that includes spin-orbit effects and also averaged relativistic EP (AREP) and a nonrelativistic EP (NREP). All-electron nonrelativistic calculations were also performed. The values of spectroscopic constants obtained from these calculations indicate that relativistic effects account for a decrease in R e of over 0.3 Å and an increase in the bond energy of the order of 1 eV. Various intercomparisons indicate the general validity of effective potential methods, properly applied, but also show certain limitations. In particular, the NREP results agree well with the all-electron, nonrelativistic calculations. Also, various relativistic effective-potential methods agree for SCF calculations provided both the basis sets and the EP are carried to sufficiently high order in angular quantum number. The bond distance calculated relativistically agrees very well with experiment. © 1979 American Institute of Physics.