  Optical Fe II emission is a strong feature in quasar spectra originating in
the broad-line region (BLR). The difficulty in understanding the complex Fe II
pseudo-continuum has led us to search for other reliable, simpler ionic species
such as Ca II. In this first part of the series, we confirm the strong
correlation between the strengths of two emission features, the optical Fe II
and the NIR Ca II, both from observations and photoionization modelling. With
the inclusion of an up-to-date compilation of observations with both optical Fe
II and NIR Ca II measurements, we span a wider and more extended parameter
space and confirm the common origin of these two spectral features with our
photoionization models using CLOUDY. Taking into account the effect of dust
into our modelling, we constrain the BLR parameter space (primarily, in terms
of the ionization parameter and local cloud density) as a function of the
strengths of Fe II and Ca II emission.
