A direct generalized Bloch approach: ground and excited states of C-2

In this study, the direct iterative algorithm for solving the generalized B loch equation [J. Chem. Phys. 113 (2000) 2594], referred to as DGB approach , is applied to the C-2 molecule. The one-determinantal (ID), respectively, two-determinantal state-specific or state-selective (2D SS) version of the DGB approach is used to calculate the potential energy curves for the sing let ground state X(1)Sigma (+)(g) and the first three excited states a (3)P i (u), b(3)Sigma (-)(g) and A(1)Pi (u). To assess the performance of the DG B method, both correlation-consistent polarized-valence double zeta (cc-pVD Z) and triple zeta (cc-pVTZ) basis sets of Gaussian-type orbitals (GTOs) ar e employed. Beside these GTO bases, new Slater-type orbital (STO) basis set s are also examined for both the ground and excited states of the C2 molecu le. To compare the effectiveness of the STO and GTO bases, especially in th e neighborhood of the equilibrium geometry, the spectroscopic constants are calculated and compared with the available experimental data. A good agree ment of computed and experimental data is found, particularly when the STO double zeta basis is employed. (C) 2001 Elsevier Science B.V. All rights re served.