To correlate the NMR measured chemical shift (δ iso ) with the DFT calculated chemical shielding (σ GIPAW ) a reference set of crystal structures with well known NMR chemical shifts was chosen. The reference set in this study for correlating the Ca NMR shifts with the calculated chemical shieldings consisted of aragonite , calcite , β -dicalcium silicate , calcium oxide , calcium hydroxide 49 and grossite . The calculated chemical shieldings (σ GIPAW ) of the reference structures with their corresponding experimental chemical shift (δ iso ) are shown in Fig- . Figure includes the DFT predicted chemical shieldings from Moudrakovski et al. , which are in good agreement with the here reported shieldings. The linear regression fit yields then the scaling factor, δ DFT = 872.41 -0.77σ GIPAW , which was used to convert the chemical shieldings to chemical shifts for the investigated (C-S-H) structures.