Perhaps I should have lead with the problem not my ill-informed solution
this input script :
boundary p p p
lattice fcc 5.376
region box block 0 3 0 3 0 3
create_box 1 box
create_atoms 1 box
mass 1 39.948
pair_style lj/cut 13.0
pair_coeff * * 0.2381 3.405
compute PE all pe/atom
variable maxPE equal max(c_PE)
thermo_style custom step pe v_maxPE
fix NVE all nve
Setting up run ...
Memory usage per processor = 1.82285 Mbytes
Step PotEng maxPE
ERROR: Mismatched compute in variable formula
which I can attribute to the use of a "special" function such as max on (any) per-atom compute, e.g. pe/atom, displace/atom.
Perhaps I am using this wrong but I suspect a bug. I would like to offer a solution but I don't have enough knowledge of what flags indicate what. I do know this operation works fine for a "fix"
The array_flag is set for computes that calculate a global
array, not per-atom arrays. Pe/atom, displace/atom, etc
calculate per-atom vectors and arrays.