The difficulties in calculation of high temperature heat capacities and discrepancies between available data are discussed in the case of the NH molecule. The source of those inaccuracies are the underlining potential energy curves. The relevant partition functions and heat capacities are calculated with the classical Wigner-Kirkwood corrected method. In addition to the ground electronic state, also four excited states are taken into account. Two potential energy curves and experimental energy levels of the ground state are considered.