Direct first principles calculations of d→d excitation energies in pure and hole-doped Sr2CuO2Cl2

Direct first principles calculations are conducted for the three spin-allowed d→d excitations in pure and hole-doped antiferromagnetic Sr2CuO2Cl2. The results obtained for the pure system are close to the resonant x-ray Raman spectra reported by Kuiper et al (1998 Phys. Rev. Lett. 80 5204), most notably in respect of the dz2 → dx2-y2 state, which was not observed directly. The energy of 1.53 eV computed for this excitation is in good agreement with the value 1.5 eV deduced from the Raman experiment, and both of these lie well above the energy 0.5 eV suggested previously on the basis of the optical spectrum (Perkins et al 1993 Phys. Rev. Lett. 71 1621). The associated spin-flip energy of approximately 0.2 eV proposed by Kuiper et al is shown to be entirely consistent with the observed Neel temperature and with first principles calculations, and further, that it corresponds to the flip of an unpaired dx2-y2 spin in the ground state rather than a dz2 spin in the excited state. The two t2g → eg excitation energies in the current UHF calculations differ by approximately 0.25 eV from the Raman values, an amount ascribed to the difference in pair correlation energies. In addition, hybrid functional calculations incorporating varying contents of exact exchange are found to offer no systematic improvement. The presence of a nearest neighbour hole in the most stable O(p) configuration is shown to have no significant effect upon either the order or the stability of the d → d states, with changes in excitation energy of 0.1-0.2 eV. A comparison with previous cluster calculations indicates that the latter do not capture fully the effect of the surrounding lattice on these highly local excitations. The generality of the direct approach to excitations is further established by calculations of the energies of three nearest neighbour charge-transfer states, which are placed in the range from 5.3 to 5.6 eV, as compared with values around 5 eV predicted previously (Tanaka and Kotani 1993 J. Phys. Soc. Japan 62 464). The d→d and charge-transfer excitations produce extensive renormalization of the valence levels that lead to substantial reductions in energy from the rigid-band estimates based on the ground state eigenvalue spectrum