The recent interest in the use of THz band, 0:1-10THz, for communications purposes stimulates development of new models for performance assessment of prospective technologies. One of the unique properties of this band is molecular noise generated by water vapor in response to absorption of electromagnetic radiation. In this paper we specify a model capturing the behavior of the molecular noise in the frequency domain and propose the associated parameters fitting algorithm. The model has a simple structure and can be used as a building block for bit error rate (BER) performance assessment.