Frozen density embedding with non- integer subsystems' particle numbers

We extend the frozen density embedding theory to non- integer subsystems' particles numbers. Different features of this formulation are discussed, with special concern for approximate embedding calculations. In particular, we highlight the relation between the non-integer particle- number partition scheme and the resulting embedding errors. Finally, we provide a discussion of the implications of the present theory for the derivative discontinuity issue and the calculation of chemical reactivity descriptors. (c) 2014 AIP Publishing LLC.