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 Purpose: Calculates the initial guess field for optimal control
          The pulse is a Gaussian times sin(omega_field*t)

 Usage  : guessfld tfinal tout fld_strgth omega_fld [frequency unit]

 The times tfinal, tout, are in fs. The field-strength,
 fld_strgth, and the  field-frequency, omega_fld, 
 are in eV, au or cm-1. The frequency unit can be
 given with an additional optional string. The default unit is au. 
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