An improved method is described for the computation of integrals involving effective core potentials. The improved method provides better scalability to higher angular momenta as well as improved speed. The new method is also applied to the determination of the minimum energy structures of Si8C12, Ge8C12, and Sn8C12, main group analogs of the Ti8C12 compounds (known as metcars). Relative energies, geometries, and vibrational frequencies are reported for several novel structures.