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


Optimized Structure

 0.000000    0.000000    2.906119
 0.000000    0.000000   -2.906119
 0.000000    1.960738    0.000000
 0.000000   -1.960738    0.000000

                    Distance matrix (angstroms):
                    1          2          3          4
     1  Ca   0.000000
     2  Ca   5.812238   0.000000
     3  Ca   3.505713   3.505713   0.000000
     4  Ca   3.505713   3.505713   3.921476   0.000000

 SCF Done:  E(RB+HF-PW91) = -2710.04211577

 E(4 atoms of Calcium) = -2710.039131 au

 Binding Energy = 0.081220 ev (0.0203 ev/atom)