  We use a real-space formulation of orbital-free DFT to study the core
energetics and core structure of an isolated screw dislocation in Aluminum.
Using a direct energetics based approach, we estimate the core size of a
perfect screw dislocation to be $\approx$ 7 $|{\bf b}|$, which is considerably
larger than previous estimates of $1-3~|{\bf b}|$ based on displacement fields.
The perfect screw upon structural relaxation dissociates into two Shockley
partials with partial separation distances of 8.2 \AA~and 6.6 \AA~ measured
from the screw and edge component differential displacement plots,
respectively. Similar to a previous electronic structure study on edge
dislocation, we find that the core energy of the relaxed screw dislocation is
strongly dependent on macroscopic deformations. Next, we use the edge and screw
dislocation core energetics data with physically reasonable assumptions to
develop a continuum energetics model for an aggregate of dislocations. Further,
we use this model in a discrete dislocation network, and from the variations of
the core energy with respect to the nodal positions of the network, we obtain
the nodal core force which can directly be incorporated into discrete
dislocation dynamics frameworks. We analyze and classify the nodal core force
into three different contributions based on their decay behavior. Two of these
contributions to the core force, both arising from the core energy dependence
on macroscopic deformations, are not accounted for in currently used discrete
dislocation dynamics models which assume the core energy to be a constant.
Using case studies involving simple dislocation structures, we demonstrate that
the contribution to the core force from the core energy dependence on
macroscopic deformations can be significant in comparison to the elastic
Peach-Koehler force even up to distances of $10-15$ nm between dislocation
structures.
