A COMPLETE BASIS SET MODEL CHEMISTRY .3. THE COMPLETE BASIS SET-QUADRATIC CONFIGURATION-INTERACTION FAMILY OF METHODS

The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. A family of complete basis set (CBS) quadratic CI (QCI) model chemistries is defined to include corrections for basis set truncation errors. These models use basis sets ranging from the small 6-31 G dagger dagger double zeta plus polarization (DZ + P) size basis set to the very large (14s9p4d 2f,6s3p1d)/[6s6p3d 2f,4s2p1d] atomic pair natural orbital basis set. When the calculated energies are compared with the experimental energies of the first-row atoms and ions and the first-row diatomics and hydrides H-2, LiH, Li-2, CH4, NH3, H2O, HF, LiF, N2, CO, NO, O2, and F2, two very promising new model chemistries emerge. The first is of comparable accuracy, but more than ten times the speed of the G1 model of Pople and co-workers. The second is less than one-tenth the speed of the G1 model, but reduces the root-mean-square (rms) errors in ionization potentials (IPs), electron affinities (EAs), and D0's to 0.033 and 0.013 eV, and 0.53 kcal/mol per bond, respectively.