  We explore the physics of SN Ia light curves and spectra using the 1-D
non-LTE time-dependent radiative-transfer code CMFGEN. Rather than adjusting
ejecta properties to match observations, we select as input one "standard" 1-D
Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore
the sensitivity of radiation and gas properties on radiative-transfer modeling
assumptions. The correct computation of SN Ia radiation is not exclusively a
solution to an "opacity problem", characterized by the treatment of a large
number of lines. It is also key to treat important atomic processes
consistently. Besides handling line blanketing in non-LTE, we show that
including forbidden line transitions of metals is increasingly important for
the temperature and ionization of the gas beyond maximum light. Non-thermal
ionization and excitation are also critical since they affect the color
evolution and the Delta-M15 of our model. While impacting little the bolometric
luminosity, a more complete treatment of decay routes leads to enhanced line
blanketing, e.g., associated with 48Ti in the U and B bands. Overall, we find
that SN Ia radiation properties are influenced in a complicated way by the
atomic data we employ, so that obtaining converged results is a challenge. We
nonetheless obtain a good match to the golden standard type Ia SN 2005cf in the
optical and near-IR, from 5 to 60d after explosion, suggesting that assuming
spherical symmetry is not detrimental to SN Ia radiative-transfer modeling at
these times. Multi-D effects no doubt matter, but they are perhaps less
important than accurately treating non-LTE processes [abridged].
