One important implementation note bears pointing out. Density matrices P(t) at each time step are not retained, as these would represent an intractable amount of storage for most calculations. As a result, while there is virtually no overhead for computing arbitrary partitions of the sum in Eq. ( ), the decision of which partitions to examine must be made at the outset. New partitions cannot be examined in post-processing. However, it would be possible to store dipole moment data µ λ,k (t) for each individual MO ψ k , in which case arbitrary partitions could be assembled after-the-fact.