Self-Consistent Molecular-Orbital Methods. III. Comparison of Gaussian Expansion and PDDO Methods Using Minimal STO Basis Sets

Self-consistent-field molecular-orbital calculations over a minimal basis set of Slater-type atomic orbitals are presented for a set of organic molecules and positive ions containing up to eight first-row atoms. The necessary molecular integrals are calculated by two previously introduced schemes: the Gaussian expansion (STO–KG) method and the projection of diatomic-differential-overlap (PDDO) method. Atomization energies, electric dipole moments, density matrices, optimum STO ζ exponents, and computation times are compared for the PDDO, STO-3G, and STO-4G methods, the latter of which has previously been shown to closely reproduce the full STO results. Relative to the STO-4G values, the PDDO method leads to errors of up to 0.22 a.u. in the atomization energy, 0.16D for the dipole moment, and 0.05 for the optimum ζ exponents. The corresponding limits for the STO-3G method are 0.06 a.u., 0.07D, and 0.02. Two electron integrals are evaluated at rates of 125–175, 25–140, and 10–70 integrals per second for the PDDO, STO-3G, and STO-4G methods, respectively, on a CDC 1604A computer.