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

Abstract

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 $\zeta$ 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, $\text{0.16D}$ for the dipole moment, and 0.05 for the optimum $\zeta$ exponents. The corresponding limits for the STO‐3G method are 0.06 a.u., $\text{0.07D}$ , 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.