Cell volume V in angstroms cubed.
V = a b c (1 - cos2alpha - cos2beta - cos2gamma
+ 2 cosalpha cosbeta cosgamma)1/2
a = _pdbx_diffrn_merged_cell.cell_length_a
b = _pdbx_diffrn_merged_cell.cell_length_b
c = _pdbx_diffrn_merged_cell.cell_length_c
alpha = _pdbx_diffrn_merged_cell.cell_angle_alpha
beta = _pdbx_diffrn_merged_cell.cell_angle_beta
gamma = _pdbx_diffrn_merged_cell.cell_angle_gamma