use precise summation

qcmanybody/tests/test_computer_het4_gradient.py

@@ -243,11 +243,10 @@ def het_tetramer():
         15,
         id="4b_nocp_rtd_ee"),
 ])
-def test_nbody_het4_grad(mbe_keywords, anskeyE, anskeyG, bodykeys, outstrs, calcinfo_nmbe, het_tetramer, request, mbe_data_grad_dtz):
+def test_nbody_het4_grad(mbe_keywords, anskeyE, anskeyG, bodykeys, outstrs, calcinfo_nmbe, het_tetramer, request, mbe_data_grad_dtz, monkeypatch):
     _inner = request.node.name.split("[")[1].split("]")[0]
     kwdsln, pattern, progln = _inner, "", "psi4"
-    print("LANE", kwdsln, pattern, progln)
-    os.environ["QCMANYBODY_EMBEDDING_CHARGES"] = "1"
+    monkeypatch.setenv("QCMANYBODY_EMBEDDING_CHARGES", "1")
 
     mbe_keywords = ManyBodyKeywords(**mbe_keywords)
     mbe_data_grad_dtz["molecule"] = het_tetramer

qcmanybody/utils.py

@@ -2,6 +2,7 @@
 
 import ast
 import json
+import math
 import re
 import string
 from typing import Any, Dict, Iterable, List, Literal, Mapping, Optional, Set, Tuple, Union
@@ -222,13 +223,22 @@ def sum_cluster_data(
     shape = find_shape(data[first_key])
     ret = shaped_zero(shape)
 
-    for frag, bas in compute_list:
-        egh = data[labeler(mc_level_lbl, frag, bas)]
-
-        if vmfc:
-            sign = (-1) ** (nb - len(frag))
-
-        ret += sign * egh
+    precise_sum_func = math.fsum if isinstance(ret, float) else np.sum
+    ret = precise_sum_func(
+        (((-1) ** (nb - len(frag))) if vmfc else 1) * (data[labeler(mc_level_lbl, frag, bas)])
+        for frag, bas in compute_list
+    )
+
+    # A more readable format for the above but not ammenable to using specialty summation functions
+    # ```
+    # for frag, bas in compute_list:
+    #     egh = data[labeler(mc_level_lbl, frag, bas)]
+    #
+    #     if vmfc:
+    #         sign = (-1) ** (nb - len(frag))
+    #
+    #     ret += sign * egh
+    # ```
 
     return ret