The calculation of accurate electron affinities (EAs) of atomic or molecular species is one of the most challenging tasks in quantum chemistry. We describe a reliable procedure for calculating the electron affinity of an atom and present results for hydrogen, boron, carbon, oxygen, and fluorine (hydrogen is included for completeness). This procedure involves the use of the recently proposed correlation-consistent basis sets augmented with functions to describe the more diffuse character of the atomic anion coupled with a straightforward, uniform expansion of the reference space for multireference singles and doubles configurationinteraction (MRSD-Cl) calculations. Comparison with previous results and with corresponding full CI calculations are given. The most accurate EAs obtained from the MRSD-CI calculations are (with experimental values in parentheses) hydrogen 0.740 eV (0.754), boron 0.258 (0.277), carbon 1.245 (1.263), oxygen 1.384 (1.461), and fluorine 3.337 (3.40 1). The EAs obtained from the MR-SDCI calculations differ by less than 0.03 e V from those predicted by the full CI calculations.
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.
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