Pure second order Stark effect:
      E.level = E.o + ( A.g + B.g M^2 ) mu.g^2 E^2
 Mixed order:
      E.level = (E1+E2)/2  +- [ (DE/2)^2 + mu.g^2 E^2 |xi|^2 M^2 ]^(1/2)

              = E1-DE/2+[...]^(1/2) and
                E2+DE/2-[...]^(1/2) for E1>E2, DE=E1-E2

 * E1,E2,DE should in general be inclusive of second order Stark contributions
 * Asterisk denotes levels implicated in near degeneracy for which
   A and B are evaluated without the connecting (|Phi.Zg|) term
 * Sum field contributions for more than one dipole component
 * Dipoles in D, energies in MHz, field in V/cm
 * In the .INT file set mu.g to 1 (only one at a time) and STRFLG=1

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  SO2                                                            
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           A.g           B.g       level                         g


   -1.214082E-06     1.214082E-06  0 0 0                         1    1

   -5.813827E-07    -1.073520E-06  1 0 1                         1    1
   -2.651729E-07     1.285159E-06  1 1 0                         1    1
    2.394917E-06    -1.509291E-06  1 1 1                         1    1

    1.342888E-07    -6.606998E-09  2 2 1                         1    1
   -1.734391E-07     3.593255E-07  2 1 1                         1    1
   -2.412199E-06     1.421310E-07  2 1 2                         1    1
    1.211625E-07    -8.774357E-10  2 2 0                         1    1
   -1.860645E-06     1.810367E-08  2 0 2                         1    1

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