Application of time-dependent density-functional theory to the (3)Sigma(-)(u) first excited state of H-2

Recently, time-dependent density-functional (TDDFT) methods have been devel oped for determining the energies of molecular excited states. This, along with the somewhat similar equations-of-motion coupled-cluster (EOM-CCSD) me thods, offer advantages of speed, reliability, and often accuracy over alte rnate complete-active-space self-consistent-field (CASSCF) based approaches , with the disadvantages associated with being essentially "single-referenc e" calculations. We compare results obtained using both approaches for the (1)Sigma(g)(+) (ground) and (3)Sigma(u)(-) (first excited) states of the si mplest molecule, H-2. For the excited state of this two-electron system, EO M-CCSD is equivalent to full configuration interaction, while results obtai ned using TDDFT are good at short bond lengths but become quite poor as the bond is stretched from its equilibrium length. The poor TDDFT result is at tributed to the fact that the spin-restricted Kohn-Sham (RKS) method used t o generate the ground-state density is not size consistent. We suggest that TDDFT calculations based on spin-unrestricted Kohn-Sham (UKS) calculations should provide better descriptions of molecular excited states than do cur rent RKS-based methods, spin-contamination effects notwithstanding. (C) 200 0 American Institute of Physics. [S0021-9606(00)31101-1].