  We have performed ab initio calculations for new high-pressure phase of Ca-VI
between 158-180 GPa. The study includes elastic parameters of mono- and
poly-crystalline aggregates, electronic band structure, lattice dynamics and
superconductivity. The calculations show that the orthorhombic Pnma structure
is mechanically and dynamically stable in the pressure range studied. The
structure is superconducting in the entire pressure range and the calculated Tc
(~25K) is maximum at ~172 GPa, where the transfer of charges from 4s to 3d may
be thought to be completed.
