  The concept of the quasar main sequence is very attractive since it stresses
correlations between various parameters and implies the underlying simplicity.
In the optical plane defined by the width of the H\{beta} line and the ratio of
the equivalent width of the Fe II to H\{beta} observed objects form a
characteristic pattern. In this paper, we use a physically motivated model to
explain the distribution of quasars in the optical plane. Continuum is modelled
as an accretion disk with a hard X-ray power law uniquely tight to the disk at
the basis of observational scaling, and the Broad Line Region distance is
determined also from observational scaling. We perform the computations of the
FeII and H\{beta} line production with the code CLOUDY. We have only six free
parameters for an individual source: maximum temperature of the accretion disk,
Eddington ratio, cloud density, cloud column density, microturbulence, and iron
abundance, and only the last four remain as global parameters in our modelling
of the whole sequence. Our theoretically computed points cover well the optical
plane part populated with the observed quasars, particularly if we allow for
super-Solar abundance of heavy elements. Explanation of the exceptionally
strong Fe II emitter requires a stronger contribution from the dark sides of
the clouds. Analyzing the way how our model covers the optical plane we
conclude that there is no single simple driver behind the sequence, as neither
the Eddington ratio nor broad band spectrum shape plays the dominant role.
Also, the role of the viewing angle in providing the dispersion of the quasar
main sequence is apparently not as strong as expected.
