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

Optimized Structure

-0.419509    1.483830    3.157005
-0.268206    3.737190    0.000000
-0.419509    1.483830   -3.157005
 2.828037   -3.141867    0.000000
-2.290001    0.572904    0.000000
-0.419509   -2.260833    1.921744
 1.408207    0.385779    0.000000
-0.419509   -2.260833   -1.921744

                    Distance matrix (angstroms):
                    1          2          3          4          5
     1  Ca   0.000000
     2  Ca   3.881649   0.000000
     3  Ca   6.314010   3.881649   0.000000
     4  Ca   6.473817   7.543748   6.473817   0.000000
     5  Ca   3.780901   3.755045   3.780901   6.324068   0.000000
     6  Ca   3.943143   6.300180   6.310007   3.875032   3.901526
     7  Ca   3.809585   3.747308   3.809585   3.802657   3.702939
     8  Ca   6.310007   6.300180   3.943143   3.875032   3.901526
                    6          7          8
     6  Ca   0.000000
     7  Ca   3.746758   0.000000
     8  Ca   3.843487   3.746758   0.000000

SCF Done:  E(RB+HF-PW91) =  -5420.26577196 au

 E(8 atoms of Calcium) = -5420.078262 au

 Binding Energy = 5.1050 ev (0.6381 ev/atom)