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

Optimized Structure

 0.000000    3.228781   -0.539620
 3.090776   -2.250678    1.397850
 0.000000    0.000000    1.563442
 0.000000   -3.228781   -0.539620
 1.957901   -0.219326   -1.639952
-3.090776    2.250678    1.397850
-1.957901    0.219326   -1.639952

                    Distance matrix (angstroms):
                    1          2          3          4          5
     1  Ca   0.000000
     2  Ca   6.582640   0.000000
     3  Ca   3.853297   3.826993   0.000000
     4  Ca   6.457561   3.776688   3.853297   0.000000
     5  Ca   4.115039   3.825969   3.760747   3.755120   0.000000
     6  Ca   3.776688   7.646817   3.826993   6.582640   6.388921
     7  Ca   3.755120   6.388921   3.760747   4.115039   3.940295
                    6          7
     6  Ca   0.000000
     7  Ca   3.825969   0.000000

 SCF Done:  E(RB+HF-PW91) =  -4742.71331480 au

 E(7 atoms of Calcium) = -4742.568480 au

 Binding Energy = 3.9412 ev (0.5630 ev/atom)