Density-functional theory investigation of the geometric, energetic, and optical properties of the cobalt(II)tris(2,2′-bipyridine) complex in the high-spin and the Jahn-Teller active low-spin states

State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D-3 [Co(bpy)(3)](2+) complex in the (4)A(2) and E-4 trigonal components of the high-spin T-4(1g)(t(2g)(5) e(g)(2)) state and in the low-spin E-2 state of octahedral E-2(g)(t(2g)(6) e(g)(1)) parentage. Their comparison extends also to the investigation of the Jahn-Teller instability of the E-2 state through the characterization of the extrema of C-2 symmetry of this spin state's potential energy surface. The results obtained for [Co(bpy)(3)](2+) in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference Delta E-HL(el) vary between -3212 and 3919 cm(-1). Semilocal functionals tend to give too large Delta E-HL(el) values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)(3)](2+) in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results.