What drives the Quasar Main Sequence?
  Eigenvector 1 (EV1) was found to be the dominant component behind the
significant correlations for the measured parameters in quasar spectra (Boroson
& Green, 1992). The parameter R$_{\mathrm{FeII}}$, which strongly correlates to
the EV1, is the ${\mathrm{FeII}}$ strength, defined to be the ratio of the
equivalent width of ${\mathrm{FeII}}$ to the equivalent width of
${\mathrm{H\beta}}$. This allows to construct a quasar main sequence analogous
to the stellar properties driven HR diagram (Sulentic et al. 2001). We try to
find the main driver behind the EV1 among the basic (theoretically motivated)
parameters of an active nucleus (Eddington ratio, black hole mass, accretion
rate, spin, and viewing angle). Based on theoretical modeling using the
photoionization code CLOUDY (Ferland et al. 2013), we test the hypothesis that
the physical driver of EV1 is the maximum of the accretion disk temperature
($\mathrm{T_{BBB}}$), reflected in the shape of the spectral energy
distribution (SED). We have assumed that both H$\mathrm{\beta}$ and
Fe${\mathrm{II}}$ emission come from the Broad Line Region represented as a
constant density cloud in a plane-parallel geometry. We test the effect of
changing Eddington ratio on the $\mathrm{R_{FeII} - T_{BBB}}$ trends with
varying mean hydrogen densities. We also test the effect of adding
microturbulence that affect the line intensities on the overall
$\mathrm{R_{FeII} - T_{BBB}}$ picture.
