code: def _write_angle_information(xml_file, structure, ref_energy):
      
  
      unique_angle_types = set()
      xml_file.write('<angle>\n')
      for angle in structure.angles:
          t1, t2, t3 = angle.atom1.type, angle.atom2.type, angle.atom3.type
          t1, t3 = sorted([t1, t3])
          angle_type = ('-'.join((t1, t2, t3)), angle.type.k, angle.type.theteq)
          unique_angle_types.add(angle_type)
          xml_file.write('{} {} {} {}\n'.format(
              angle_type[0], angle.atom1.idx, angle.atom2.idx, angle.atom3.idx))
      xml_file.write('</angle>\n')
      xml_file.write('<angle_coeffs>\n')
      xml_file.write('<!-- type k theta_eq -->\n')
      for angle_type, k, teq in unique_angle_types:
          xml_file.write('{} {} {}\n'.format(angle_type,
              k * 2.0 / ref_energy, radians(teq)))
      xml_file.write('</angle_coeffs>\n')
docs: Write the angles in the system.
  
      Parameters
      ----------
      xml_file : file object
          The file object of the hoomdxml file being written
      structure : parmed.Structure
          Parmed structure object
      ref_energy : float, default=1.0
          Reference energy for conversion to reduced units