--- a/+sbp/+implementations/d2_noneq_variable_4.m	Fri Oct 14 14:42:32 2022 +0200
+++ b/+sbp/+implementations/d2_noneq_variable_4.m	Sat Oct 15 15:48:20 2022 +0200
@@ -100,6 +100,8 @@
 
     % Helper functions for constructing D2(c)
     % TODO: Consider changing sparse(diag(...)) to spdiags(....)
+    min_inds = sbp.implementations.d2_sparsity_pattern_inds(m, order, BP, 0, 2);
+    nonmin_inds = sbp.implementations.d2_sparsity_pattern_inds(m, order, BP, 1, 2);
 
     % Minimal 5 point stencil width
     function D2 = D2_fun_minimal(c)
@@ -113,6 +115,11 @@
         % point narrow stencil.
         R = (1/144 / h) * transpose(DD_4) * C1 * DD_4 + (1/18 / h) * transpose(DD_3) * C2 * DD_3;
         D2 = D1 * C1 * D1 - H \ R;
+
+        % Remove potential round off zeros
+        D2tmp = sparse(m,m);
+        D2tmp(min_inds) = D2(min_inds);
+        D2 = D2tmp;
     end
 
     %  Non-minimal 7 point stencil width
@@ -123,6 +130,11 @@
         % Remainder term added to wide second derivative operator
         R = (1/144 / h) * transpose(DD_4) * C1 * DD_4;
         D2 = D1 * C1 * D1 - H \ R;
+        
+        % Remove potential round off zeros
+        D2tmp = sparse(m,m);
+        D2tmp(nonmin_inds) = D2(nonmin_inds);
+        D2 = D2tmp;
     end
 
     % Wide stencil