Calculate NEB force for 1 image. That contains projected real force and spring force.
The current implementation is the DNEB (doubly nudged elastic band) as described in
“A doubly nudged elastic band method for finding transition states” Semen A. Trygubenko and David J. Wales J. Chem. Phys. 120, 2082 (2004); doi: 10.1063/1.1636455