An ab initio treatment of the electronic absorption spectra of excess-electron alkali halide clusters Nan+1Cln up to Na18Cl17

Electronic excitation energies of alkali-excess clusters Na2Cl, Na3Cl2, Na4Cl3, Na6Cl5, Na14Cl13, and Na18Cl17 are investigated using CIS (configuration interaction singles) and RPA (random phase approximation). The accuracy of these approximations is established for Na2Cl by equation-of-motion coupled-cluster singles and doubles calculations and by comparison to experimental results. The mode of localization of the excess electron is decisive for the electronic excitation energy. No cluster-size dependence of the excitation energy is found. The direct UHF-CIS (unrestricted Hartree–Fock-CIS) and UHF-RPA implementation within the program package TURBOMOLE is briefly described in the appendix.