Before we dive into the Quantum Crystallographic investigation of this system, let's begin with a general survey of the phase transitions that lead to Ca-sc (Ca-III) and familiarize ourselves with the thermodynamics of calcium phases as a function of pressure (Figure ). Upon compression, Ca follows the fcc → bcc → sc phase transitions. Between 300 -750 K, the first phase transition from fcc to bcc is observed at ~ 20 GPa, while the second one, from bcc to sc, at ~35 -40 GPa . Our calculated enthalpies (H = E + PV), obtained at 0 K using the meta-GGA exchange-correlation R 2 SCAN functional , and in the static lattice approximation (See Computational Details for further information), predict that bcc becomes thermodynamically the most stable phase at 10 GPa, while sc is preferred at 26 GPa (Figure ). These stability boundaries are systematically lower in pressure compared to the experimental ones. However, zero-point energy corrections and finite temperature effects can easily shift the pressure at which a phase emerges as being the most stable in calculations, and kinetic barriers may inhibit its appearance in experiments. To this point, bcc is observed already at 5 -10 GPa for T of ~ 800 K . Therefore, we can consider our calculations in quite good agreement with the experimental observations. To be noted, a tetragonal phase having space group I41/amd, and an orthorhombic phase, with space group Cmmm (identified as a distorted Ca-sc), were also observed at low temperatures between 35 -40 GPa , but we will not discuss these phases in our analyses.