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

--- Lowest Energy Structure ---

Optimized Geometry

 0.000000    2.050980    2.093274
 0.000000    3.312959   -1.524697
 1.809339    0.000000   -0.568577
-1.809339    0.000000   -0.568577
 0.000000   -3.312959   -1.524697
 0.000000   -2.050980    2.093274

                    Distance matrix (angstroms):
                    1          2          3          4          5
     1  Ca   0.000000
     2  Ca   3.831749   0.000000
     3  Ca   3.816501   3.894043   0.000000
     4  Ca   3.816501   3.894043   3.618678   0.000000
     5  Ca   6.470051   6.625919   3.894043   3.894043   0.000000
     6  Ca   4.101961   6.470051   3.816501   3.816501   3.831749
                    6
     6  Ca   0.000000

 SCF Done:  E(RB+HF-PW91) =  -4065.17198676 au

 E(6 atoms of Calcium) = -4065.058697 au

 Binding Energy = 3.0828 ev (0.5138 ev/atom)


 Single Point Energy Calculation
 b3pw91/6-311+g(df)
 SCF Done:  E(RB+HF-PW91) =  -4065.173241 Hartrees
 E(6 atoms of Calcium) = -4065.058876 Hartrees
 B.E. = 0.5186749 ev