Using Gaussian98
#b3pw91/6-311g(d) scf=(direct,dsymm,symm) symm=loose fopt


Optimized Structure

 0.000000    2.064090    0.000000
 0.000000   -2.064090    0.000000
 0.001969    6.249161    0.000000
-0.001969   -6.249161    0.000000

                    Distance matrix (angstroms):
                    1          2          3          4
     1  Ca   0.000000
     2  Ca   4.128180   0.000000
     3  Ca   4.185071   8.313251   0.000000
     4  Ca   8.313251   4.185071  12.498322   0.000000

 SCF Done:  E(RB+HF-PW91) =  -2710.05961525 au

 E(4 atoms of Calcium) = -2710.039131 au

 Binding Energy = 0.5574 ev (0.1393 ev/atom)