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The concept of the quasar main sequence is very attractive since it stressescorrelations between various parameters and implies the underlying simplicity.In the optical plane defined by the width of the H\{beta} line and the ratio ofthe equivalent width of the Fe II to H\{beta} observed objects form acharacteristic pattern. In this paper, we use a physically motivated model toexplain the distribution of quasars in the optical plane. Continuum is modelledas an accretion disk with a hard X-ray power law uniquely tight to the disk atthe basis of observational scaling, and the Broad Line Region distance isdetermined also from observational scaling. We perform the computations of theFeII and H\{beta} line production with the code CLOUDY. We have only six freeparameters for an individual source: maximum temperature of the accretion disk,Eddington ratio, cloud density, cloud column density, microturbulence, and ironabundance, and only the last four remain as global parameters in our modellingof the whole sequence. Our theoretically computed points cover well the opticalplane part populated with the observed quasars, particularly if we allow forsuper-Solar abundance of heavy elements. Explanation of the exceptionallystrong Fe II emitter requires a stronger contribution from the dark sides ofthe clouds. Analyzing the way how our model covers the optical plane weconclude that there is no single simple driver behind the sequence, as neitherthe Eddington ratio nor broad band spectrum shape plays the dominant role.Also, the role of the viewing angle in providing the dispersion of the quasarmain sequence is apparently not as strong as expected.

Modelling of the quasar main sequence in the optical plane