A complete basis set model chemistry. III. 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°° double zeta plus polarization (DZ+P) size basis set to the very large (14s9p4d2f,6s3p1d)/[6s6p3d2f,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 H2, LiH, Li2, 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.