[en] The calculation of intensities of infrared vibrational transitions and of Raman scattering due to molecular vibrations raises theoretical problems of great interest. The calculation of infrared inten sities might seem at first sight always to be the simpler of these problems, as it involves the gradient of the expectation value of a first-order ground state property only. In practice, this is not always the case, particularly at the level of correlated calculation. Useful upper or lower bounds cannot be set for either property except in special cases which are not of general practical interest. The energy-efficient basis sets employed for ground-state energy calculations are inadequate for the estimation of either dipole moment or polarisability gradients, and determination of the factor governing the choice of appropriate bases is of fundamental importance