text: import psi4
  import optking
  import numpy as np
  import pytest
  
  # Test interfragment coordinate B matrix numerically.
  @pytest.mark.dimers
  def test_dimers_bmat():
      h2oA = psi4.geometry(
          """
           O
           H 1 1.0
           H 1 1.0 2 104.5
      """
      )
      Axyz = h2oA.geometry().np
      h2oB = psi4.geometry(
          """
           O
           H 1 1.0
           H 1 1.0 2 104.5
      """
      )
      Bxyz = Axyz.copy() + 5.0  # Move B not right on top of A.
  
      dimer = {
          "Natoms per frag": [3, 3],
          "A Frag": 1,
          "A Ref Atoms": [[1], [2, 3], [3]],
          "B Frag": 2,
          "B Ref Atoms": [[4], [5, 6], [6]],
      }
      Itest = optking.dimerfrag.DimerFrag.fromUserDict(dimer)
  
      # Here is lower level method
      # Aref = [[0],[1,2],[2]]
      # Bref = [[0],[1,2],[2]]
      # Itest = optking.dimerfrag.DimerFrag(0,Aref,1,Bref)
  
      Itest.update_reference_geometry(Axyz, Bxyz)
      print(Itest)
  
      max_error = Itest.test_B(Axyz, Bxyz)
  
      print("Max. difference between analytic and numerical B matrix: {:8.3e}".format(max_error))
      assert max_error < 1.0e-9
  