The performance of G2(MP2) and G2(MP2,SVP) theories for molecules containing third-row nontransition elements Ga-Kr is assessed. The average absolute deviation from experiment for 40 test energies is 1.92 kcal/mol for both methods compared to 1.37 kcal/mol for G2 theory. Four density functional theories (BPW91, BLYP, B3PW91, and B3LYP) are also assessed for the 40 test energies and found to have average absolute deviations of 3.58, 4.75, 2.03, and 2.62 kcal/mol, respectively. The B3PW91 density functional theory gives the best agreement with experiment in contrast to first-and second-row systems, where B3LYP does better than B3PW91. Of the four density functional methods, the B3PW91 method gave the best agreement with experiment for geometries and is about as accurate as second-order perturbation theory.