Showing papers by "Carmen Herrmann published in 2005"

Comparative analysis of local spin definitions

Abstract: This work provides a survey of the definition of electron spin as a local property and its dependence on several parameters in actual calculations. We analyze one-determinant wave functions constructed from Hartree-Fock and, in particular, from Kohn-Sham orbitals within the collinear approach to electron spin. The scalar total spin operators Ŝ2 and Ŝz are partitioned by projection operators, as introduced by Clark and Davidson, in order to obtain local spin operators ŜA⋅ŜB and ŜzA, respectively. To complement the work of Davidson and co-workers, we analyze some features of local spins which have not yet been discussed in sufficient depth. The dependence of local spin on the choice of basis set, density functional, and projector is studied. We also discuss the results of Ŝz partitioning and show that 〈ŜzA〉 values depend less on these parameters than 〈ŜA⋅ŜB〉 values. Furthermore, we demonstrate that for small organic test molecules, a partitioning of Ŝz with preorthogonalized Lowdin projectors yields nearly the same results as one obtains using atoms-in-molecules projectors. In addition, the physical significance of nonzero 〈ŜA⋅ŜB〉 values for closed-shell molecules is investigated. It is shown that due to this problem, 〈ŜA⋅ŜB〉 values are useful for calculations of relative spin values, but not for absolute local spins, where 〈ŜzA〉 values appear to be better suited.

Theoretical study on the spin-state energy splittings and local spin in cationic [Re]-Cn-[Re] complexes.

Abstract: Total spin-state energy splittings are calculated for mono- and dications of the formula {[Re]−Cn−[Re]}z+ where [Re] = η5-(C5Me5)Re(NO)(PPh3). Cn is an even-numbered carbon chain with n ranging from 4 to 20, and z is 1 or 2. These complexes are experimentally known, and their potential role as molecular electronic devices initiated this work. We have considered the different total spin states monocation/doublet, monocation/quartet, dication/singlet, and dication/triplet. Data obtained for two density functionals BP86 and B3LYP* were compared to verify the internal consistency of the results. In both ionization states, the low-spin state is the ground state, but the spin-state splittings decrease as the chain gets longer. For the dications, the splitting reaches a nearly constant value of about 10 kJ/mol with BP86 and about 4 kJ/mol with B3LYP* when there are at least 14 carbon atoms in the chain, whereas for the monocations, no constant value appears to be reached asymptotically, not even if 20 carbon ato...