Extended basis sets Double- bases

Equilibrium geometries calculated at the split valence level almost always show shorter bond distances than the minimum-basis-set results and are generally in better agreement with experiment. Split valence wave functions also give considerably better electron distributions, as evidenced by dipole moments and molecule-atom electron-density differences that compare better with experiment (Hehre et al., 1986 Szabo and Ostlund, 1989). 

---

The tight-binding band structure calculations were based upon the effective one-electron Hamiltonian of the extended Huckel method. [5] The off-diagonal matrix elements of the Hamiltonian were calculated acording to the modified Wolfsberg-Helmholtz formula. All valence electrons were explicitly taken into account in the calculations and the basis set consisted of double- Slater-type orbitals for C, O and S and a single- Slater-type orbitals for H. The exponents, contraction coefficients and atomic parameters were taken from previous work [6],... 

High-accuracy calculations clearly require very flexible basis sets extended in a number of ways such as with multiple polarization sets, with diffuse basis function augmentation, and with other than atom-centered functions. Use of smaller bases goes along with less reliability however, with bases smaller than double-zeta in the valence plus one well-chosen set of polarization functions on all centers, including hydrogens, the reliability is so limited that results are not likely to be meaningful for most contemporary problems of weak interaction. 

---