
This example shows how to use pw.x to optimize crystal structures at two 
pressures for As. Arsenic is well known to undergo a phase transition from
A7 structure to imple cubic at about 30 GPa. (See da Silva CR, Wentzcovitch RM,
COMPUTATIONAL MATERIALS SCIENCE 8 (3): 219-227 JUL 1997). 

1) make a vc-relax calculation with external (target) pressure = 0 kbar, using 
   Wentzcovitch dynamics (cell_dynamics = 'damp-w' in CELL namelist), 
   (input=As.vcs00.in, output=As.vcs00.out). In this case, the angle between 
   cell vectors at the end of the calculation is 58 degrees and the internal 
   coordinate is 0.2723, typical of A7 structure.

2) make a vc-relax calculation with external (target) pressure = 500 kbar, also
   using Wentzcovitch dynamics. (input=As.vcs500.in, output=As.vcs500.out). In 
   this case, the angle between cell vectors at the end of the calculation is 
   60 degrees and the internal coordinate is 0.25,  typical of sc structure.

Both calculations start from the same initial structure with an angle of 
60 degrees between vectors and internal coordinate 0.290010 .
PLEASE NOTE: the structure has "ibrav=-0", that is, it is read from cards
"CELL_PARAMETERS". Only the lattice parametr "A" is used; the other
cell parameters B, C, COSAB COSAC, COSBC are reported for convenience, 
but thay are neither read nor used


