  Quasars are among the most luminous sources characterized by their broad band
spectra ranging from radio through optical to X-ray band, with numerous
emission and absorption features. Using the Principal Component Analysis (PCA),
Boroson & Green (1992) were able to show significant correlations between the
measured parameters. Among the significant correlations projected, the leading
component, related to Eigenvector 1 (EV1) was dominated by the anti-correlation
between the Fe${\mathrm{II}}$ optical emission and [OIII] line where the EV1
alone contained 30% of the total variance. This introduced a way to define a
quasar main sequence, in close analogy to the stellar main sequence in the
Hertzsprung-Russel (HR) diagram (Sulentic et. al 2001). Which of the basic
theoretically motivated parameters of an active nucleus (Eddington ratio, black
hole mass, accretion rate, spin, and viewing angle) is the main driver behind
the EV1 yet remains to be answered. We currently limit ourselves to the optical
waveband, and concentrate on theoretical modelling the Fe${\mathrm{II}}$ to
H$\mathrm{\beta}$ ratio, and test the hypothesis that the physical driver of
EV1 is the maximum of the accretion disk temperature, reflected in the shape of
the spectral energy distribution (SED). We performed computations of the
H$\mathrm{\beta}$ and optical Fe${\mathrm{II}}$ for a broad range of SED peak
position using CLOUDY photoionisation code. We 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
compare the results for two different approaches: (1) considering a fixed
bolometric luminosity for the SED; (2) considering $\mathrm{L_{bol}/L_{Edd}}$ =
1.
