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

 Optimized Geometry

 2.110421   -3.611289    0.000207
 2.073498    3.632586    0.000197
-4.182954   -0.021317    0.000207
-2.073519   -3.632613   -0.000211
 4.182988    0.021315   -0.000200
-2.110434    3.611317   -0.000201

                   Distance matrix (angstroms):
                    1          2          3          4          5
     1  Ca   0.000000
     2  Ca   7.243969   0.000000
     3  Ca   7.245307   7.245289   0.000000
     4  Ca   4.183994   8.365457   4.182245   0.000000
     5  Ca   4.182266   4.182251   8.366051   7.245348   0.000000
     6  Ca   8.365503   4.183986   4.182269   7.244023   7.245362
                    6
     6  Ca   0.000000

 SCF Done:  E(RB+HF-PW91) =  -4065.09796905 au

 E(6 atoms of Calcium) = -4065.058697 au

 Binding Energy = 1.06866 ev (0.17811 ev/atom)