Effective core potentials (ECPs) augmented with large, flexible valence basis sets (tested on HAt) have been used to obtain the geometries and harmonic vibrational frequencies of the diastatide compounds BeAt2, BAt2 and CAt2 for the first time. For CAt2, the ground electronic state is calculated to be the (A) over tilde (1)A(1) state, and the lowest singlet-triplet gap is calculated to be larger than that of CI2, which is against expectations based upon electronegativity.