We propose a modification to the popular 6-31G* basis set, which has recently been extended to cover first-row transition metals [Rassolov et al., J. Chem. Phys. 109, 1223 (1998)]. As demonstrated by a number of calculations, the existing basis performs poorly for many transition metals, particularly those toward the end of the series (Co, Ni, and especially Cu). The reason for this lies primarily with the 3D shell, which lacks a sufficiently diffuse exponent. A reoptimization of the D-shell exponents and coefficients by a two-step procedure, keeping the rest of the basis unchanged, corrects the problem, giving a basis set that performs uniformly well across the entire first-row transition metal series from scandium to copper.

A two-component quasirelativistic Hamiltonian based on spin-dependent effective core potentials is used to calculate ionization energies and electron affinities of the heavy halogen atom bromine through the superheavy element 117 (eka-astatine) as well as spectroscopic constants of the homonuclear dimers of these atoms. We describe a two-component Hartree-Fock and density-functional program that treats spin-orbit coupling self-consistently within the orbital optimization procedure. A comparison with results from high-order Douglas-Kroll calculations--for the superheavy systems also with zeroth-order regular approximation and four-component Dirac results--demonstrates the validity of the pseudopotential approximation. The density-functional (but not the Hartree-Fock) results show very satisfactory agreement with theoretical coupled cluster as well as experimental data where available, such that the theoretical results can serve as an estimate for the hitherto unknown properties of astatine, element 117, and their dimers.

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ABSTRACT:The X 1 + g curves of He 2 and Be 2 have been calculated by extrapolating the BSSE corrected MRCI total energies obtained with large Gaussian basis sets, large reference configuration spaces, and pseudo-natural molecular orbitals to an infinite basis. The direct calculated He 2 nonrelativistic dissociation energies (D e ) of 11.0031 K is in excellent agreement with the recent theoretical evaluations, whereas the Be 2 nonrelativistic D e = 822 cm −1 and relativistically corrected D e = 818 cm −1 are in good agreement with most known values including the experimental D e = 790 ± 30 cm −1 . An analysis of the configuration structure of Be 2 wave function and calculated vibration spectrum displays unusual type of chemical bond in this molecule and explains a unique form of the Be 2 potential curve. Thus, near the equilibrium point the bond can be classified as a conventional covalent bond, whereas at larger distances, it can be classified as van der Waals interaction. The problems of high precision Be 2 potential curve calculations are also discussed.

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