ATOM SI 0.0000000  0.000000  0.00
ATOM O  0.0000000  0.000000  1.65
ATOM O  0.0000000  1.555635 -0.55
SYMM -0.5*x - 0.86603*y, 0.86603*x - 0.5*y, z
SYMM -0.5*x + 0.86603*y, -0.86603*x - 0.5*y, z
end