def gamma_hitran(P, T, Pself, n_air, gamma_air_ref, gamma_self_ref):
    """gamma factor by a pressure broadening 
    
    Args:
       P: pressure (bar)
       T: temperature (K)
       Pself: partial pressure (bar)
       n_air: coefficient of the  temperature  dependence  of  the  air-broadened halfwidth
       gamma_air_ref: gamma air 
       gamma_self_ref: gamma self 

    Returns:
       gamma: pressure gamma factor (cm-1) 

    """
    Patm=1.01325 #atm (bar)
    Tref=296.0 #reference tempearture (K)
    gamma=(Tref/T)**n_air *(gamma_air_ref*((P-Pself)/Patm) + gamma_self_ref*(Pself/Patm))
    return gamma