@Conley11; @Suzuki12], understanding SN Ia spectral features is important for precise cosmological measurements.

SN spectra also provide detailed information about the SN explosion, progenitor composition, circumbinary environment, and reddening law [e.g., @Hoflich98; @Lentz01; @Mazzali05:hvf; @Tanaka08; @Wang09:2pop; @Foley12:csm; @Hachinger12:10jn; @Roepke12]. Furthermore, there is evidence that one can estimate the intrinsic colour of SNe Ia, and thus improve distance measurements through a better estimate of the dust reddening by measuring the ejecta velocity of SNe Ia [@Foley11:vel]. Ejecta velocity is measured from the blueshifted position of spectral features. For both cosmology and SN physics, it is important to have a precise understanding of SN Ia SEDs.

At optical wavelengths, the two most prominent features in a maximum-light spectrum of a SN Ia are at [$\sim\!\!$ ]{}3750 and 6100 Å, respectively. The latter is thought to be from [Si$\,$]{} $\lambda\lambda 6347$, 6371 ($gf$-weighted rest wavelength of 6355 Å), and is the hallmark spectral feature of a SN Ia. The former, at rest-frame wavelengths of [$\sim\!\!$ ]{}3500 – 4000 Å, is generally attributed to blueshifted absorption from Ca H&K ($gf$-weighted rest wavelength of 3945 Å). However, the line profile of this feature is complicated, often times displaying shoulders, a flat bottom, a “split” profile, 