diff --git a/examples/snap/fnum.py b/examples/snap/fnum.py
index 83bdbba503..1e50e49b09 100644
--- a/examples/snap/fnum.py
+++ b/examples/snap/fnum.py
@@ -1,8 +1,5 @@
 import lammps
 
-def sqerr(a,b):
-    return (a[0]-b[0])**2+(a[1]-b[1])**2+(a[2]-b[2])**2
-
 infile = "in.fnum"
 
 fdeltalist = [1.0e-2,1.0e-3,1.0e-4,1.0e-5,1.0e-6,1.0e-7,1.0e-8,1.0e-9,1.0e-10]
@@ -12,13 +9,6 @@ for fdelta in fdeltalist:
     cmdlist = ["-screen","none","-var","fdelta",f'{fdelta}']
     lmp = lammps.lammps(cmdargs = cmdlist)
     lmp.file(infile)
-    nlocal = lmp.extract_global("nlocal")
-
-    f = lmp.extract_atom("f")
-    fnum = lmp.extract_fix("fnum", lammps.LMP_STYLE_ATOM, lammps.LMP_TYPE_ARRAY)
-
-    sumsq = 0
-    for i in range(nlocal):
-        sumsq += sqerr(fnum[i],f[i])
-    rmse = (sumsq/nlocal)**0.5
+    faverrsq = lmp.extract_compute("faverrsq", lammps.LMP_STYLE_GLOBAL, lammps.LMP_TYPE_SCALAR)
+    rmse = faverrsq**0.5
     print(f"{fdelta} {rmse}")
diff --git a/examples/snap/in.fnum b/examples/snap/in.fnum
index addb9827d0..1298e9aed9 100644
--- a/examples/snap/in.fnum
+++ b/examples/snap/in.fnum
@@ -54,10 +54,8 @@ pair_style sna/grid grid ${ngrid} ${ngrid} ${ngrid} &
 pair_coeff * * Al
 
 fix fnum all numdiff 1 ${fdelta}
-variable fz1 equal fz[1]
-variable fnumz1 equal f_fnum[1][3]
-variable ferrz1 equal f_fnum[1][3]-fz[1]
-thermo_style custom v_fz1 v_fnumz1 v_ferrz1
-thermo_modify norm yes
+variable ferrsq atom (fx-f_fnum[1])^2+(fy-f_fnum[2])^2+(fz-f_fnum[3])^2
+compute faverrsq all reduce ave v_ferrsq
+thermo_style custom c_faverrsq
 
 run 0