update alphas covmat to include second order derivatives

validphys2/src/validphys/theorycovariance/construction.py

@@ -237,8 +237,14 @@ def covmat_alphas(name1, name2, deltas1, deltas2):
     variation given two dataset names and collections of the
     alpha_s shifts. This is equivalent to 3 point factorisation
     scale variation because it's fully correlated across all 
-    processes."""
-    return covmat_3fpt(name1, name2, deltas1, deltas2)
+    processes.
+
+    NOTE: an edit has been made to redefine the covmat to account for
+    second order derivatives of the theory prediction wrt alpha_s. (see
+    section 1.1 of 2105.05114)
+    """
+    s = 0.5 * np.outer(deltas1[0] - deltas1[1], deltas2[0] - deltas2[1])
+    return s
 
 def covmat_3fpt(name1, name2, deltas1, deltas2):
     """Returns theory covariance sub-matrix for 3pt factorisation