Proteins have characteristic circular dichroism spectra in the far-ultraviolet, depending on their secondary structure content. Perhaps the most distinctive spectrum is that of alpha-helical proteins, with an intense positive band centered about 190 nm and a negative, double-peaked band with minima at 208 and 220 nm. Traditionally, calculations of such spectra from first principles have involved parametrizations of the charge distributions associated with the electronic states and transitions of the constituent chromophoric groups. The amide group is the most important of these chromophores. In this study, using: solution phase ab initio parametrizations of the amide chromophore, we present first-principles calculations bf protein circular dichroism. Over a set of 29 proteins, there is a significant correlation between the calculated and measured intensities at 190. 208, and 220 nm. The agreement is highest at 220 nm, with a Spearman rank correlation coefficient of 0.90. This near-quantitative accuracy has allowed us to investigate, with some confidence, the dependence of the intensity at 220 nm on helix length and backbone conformation fora number of real and model helices. In this study, a better understanding of the electronic structure of-amides and improved calculations and parametrizations of the relevant charge distributions has led to significantly more accurate protein circular dichroism calculations.