check: Ewald -0.224421 Ha (bcc Madelung -0.89593/r_s = -0.224421); E_bs -0.015211
energy route at fixed volume: C' = 0.862 GPa, C44 = 6.579 GPa  (long-wave route, sodium_crystal2: C' = 0.85, C44 = 6.57)
