Time-dependent density functional theory (TDDFT) has been applied to study core excitations from Is and 2p Mn orbitals in a series of manganese complexes with oxygen and nitrogen donor ligands. The effect of basis set and functional on the excitation energy was evaluated in detail for one complex, Mn(acaC)(2) center dot (H2O)(2). The results obtained for a range of compounds, namely, [Mn(Im)(6)]Cl-2, Mn(CH3COO)(2)center dot 4H(2)O, Mn(acaC)(3), Mn(SALADHP)(2) and [Mn(SALPN)O](2), show good consistency with the data from X-ray absorption spectroscopy (XAS), confirming the relation between the Mn K-edge energy and the oxidation state of the Mn atom. The energies predicted for 2p core excitations show a dependence on the metal oxidation state very similar to that determined experimentally by 1s2p resonant inelastic X-ray scattering (RIXS) studies for Mn(acac)(2)center dot(H2O)(2), Mn(acac)(3), and Mn(sal)(2)(bipy). The reliability of the K-edge energies obtained in the present study indicates that TDDFT can be used in determining the oxidation states of Mn atoms in different computational models of the manganese cluster of photosystem II (PSII).