For C and O NMR, the principal components of the chemical shielding tensor (σ 11 , σ 22 and σ 33 where σ 33 ≥ σ 22 ≥ σ 11 ) were obtained directly from the CASTEP calculations, in terms of σ xx , σ yy and σ zz where | σ zz -σ iso | ≥ | σ xx -σ iso | ≥ | σ yy -σ iso |. The principal components of the chemical shielding tensor were converted to chemical shift principal components using δ = -(σ calc -σ ref ) where the reference values for C and 17 O were 171.2 and 249.8 ppm, respectively. These values were obtained from CASTEP calculations on cocaine ( 13 C) and the amino acids tyrosine and valine ( 17 O), and correlation of the calculated values with the experimental values with a linear fit with a fixed gradient of -1.