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

Optimized Structure

 0.000000    3.321972    0.000000
-1.952606   -2.687532    0.000000
 3.159383    1.026546    0.000000
 0.000000    0.000000    1.795856
-3.159383    1.026546    0.000000
 0.000000    0.000000   -1.795856
 1.952606   -2.687532    0.000000

                    Distance matrix (angstroms):
                    1          2          3          4          5
     1  Ca   0.000000
     2  Ca   6.318766   0.000000
     3  Ca   3.905212   6.318766   0.000000
     4  Ca   3.776320   3.776320   3.776320   0.000000
     5  Ca   3.905212   3.905212   6.318766   3.776320   0.000000
     6  Ca   3.776320   3.776320   3.776320   3.591712   3.776320
     7  Ca   6.318766   3.905212   3.905212   3.776320   6.318766
                    6          7
     6  Ca   0.000000
     7  Ca   3.776320   0.000000

 SCF Done:  E(RB+HF-PW91) =  -4742.72870442 Hartrees

 E(7 atoms of Calcium) = -4742.568480 Hartrees

 Binding Energy = 4.3625 ev (0.6232 ev/atom)