              Cell volume V in angstroms cubed.\
\
               V = a b c (1 - cos^2^~alpha~ - cos^2^~beta~ - cos^2^~gamma~\
                          + 2 cos~alpha~ cos~beta~ cos~gamma~)^1/2^\
\
               a     = _cell.length_a\
               b     = _cell.length_b\
               c     = _cell.length_c\
               alpha = _cell.angle_alpha\
               beta  = _cell.angle_beta\
               gamma = _cell.angle_gamma