Correlation consistent and augmented correlation consistent basis sets have been determined for the second row atoms aluminum through argon. The methodology, originally developed for the first row atoms [T. H. Dunning, Jr., J. Chem. Phys. 90, (1989)] is first applied to sulfur. The exponents for the polarization functions (dfgh) are systematically optimized for a correlated wave function (HF+ 1+2). The (sp) correlation functions are taken from the appropriate HF primitive sets; it is shown that these functions differ little from the optimum functions. Basis sets of double zeta [4s3pld], triple zeta [5s4p2dlf], and quadruple zeta [6s5p3d2flg] quality are defined. Each of these sets is then augmented with diffuse functions to better describe electron affinities and other molecular properties: s and p functions were obtained by optimization for the anion HF energy, while an additional polarization function for each symmetry present in the standard set was optimized for the anion HF + I+ 2 energy. The results for sulfur are then used to assist in determining double zeta, triple zeta, and quadruple zeta basis sets for the remainder of the second row of the p block.

The correlation-consistent polarized valence basis sets ͑cc-pVXZ͒ for the atoms boron through neon have been extended to treat core and core-valence correlation effects. Basis functions were added to the existing cc-pVXZ sets to form correlation-consistent polarized core-valence sets ͑cc-pCVXZ͒ in the usual pattern: Double zeta added (1s1 p), triple zeta added (2s2 p1d), quadruple zeta added (3s3p2d1 f), and quintuple zeta added (4s4 p3d2 f 1g). The exponents of the core functions were determined by minimizing the difference between all-electron and valence-only correlation energies obtained from HFϩ1ϩ2 calculations on the ground states of the atoms. With the cc-pCVXZ sets, core, core-valence, and valence correlation energies all converge exponentially toward apparent complete basis set ͑CBS͒ limits, as do the corresponding all-electron singles and doubles CI energies. Several test applications of the new sets are presented: The first two ionization potentials of boron, the 3 P-5 S separation in carbon, and the X 3 B 1-ã 1 A 1 state separation in CH 2. As expected, correlation effects involving the core electrons of the first row atoms, B-Ne, are small but must be included if high accuracy is required.

An accurate description of the electrical properties of atoms and molecules is critical for quantitative predictions of the nonlinear properties of molecules and of long-range atomic and molecular interactions between both neutral and charged species. We report a systematic study of the basis sets required to obtain accurate correlated values for the static dipole (α1), quadrupole (α2), and octopole (α3) polarizabilities and the hyperpolarizability (γ) of the rare gas atoms He, Ne, and Ar. Several methods of correlation treatment were examined, including various orders of Moller–Plesset perturbation theory (MP2, MP3, MP4), coupled-cluster theory with and without perturbative treatment of triple excitations [CCSD, CCSD(T)], and singles and doubles configuration interaction (CISD). All of the basis sets considered here were constructed by adding even-tempered sets of diffuse functions to the correlation consistent basis sets of Dunning and co-workers. With multiply-augmented sets we find that the electrical properties of the rare gas atoms converge smoothly to values that are in excellent agreement with the available experimental data and/or previously computed results. As a further test of the basis sets presented here, the dipole polarizabilities of the F− and Cl− anions and of the HCl and N2 molecules are also reported.

Using systematic sequences of correlation consistent Gaussian basis sets from double to sextuple zeta quality, the classical barrier height of the H+H2 exchange reaction has been calculated by multireference configuration interaction (MRCI) methods. The MRCI calculations for collinear H3 have also been calibrated against large basis set full CI (FCI) results, which demonstrate that the MRCI treatment leads to energies less than 1 μhartree (≤0.001 kcal/mol) above the FCI energies. The dependence of both the H2 and H3 total energies on the basis set is found to be very regular, and this behavior has been used to extrapolate to the complete basis set (CBS) limits. The resulting estimate of the H–H–H CBS limit yields a classical barrier height, relative to exact H+H2, of 9.60±0.02 kcal/mol; the best directly calculated value for the barrier is equal to 9.62 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo calculations.

Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions ͑although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms͒. After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/͓5s4p2d͔; cc-pVTZ: (20s13p9d1 f)/͓6s5p3d1 f ͔; cc-pVQZ: (21s16p12d2 f 1g)/ ͓7s6p4d2 f 1g͔; cc-pV5Z: (26s17p13d3 f 2g1h)/͓8s7p5d3 f 2g1h͔. Augmented sets were obtained by adding diffuse functions to the above sets ͑one for each angular momentum present in the set͒, with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.