Energy function for hexagonal lattice.

Terms

a(0) =3a(1) =6a(2) =12a(3) =24a(4) =54a(5) =138a(6) =378a(7) =1080a(8) =3186a(9) =9642a(10) =29784a(11) =93552a(12) =297966a(13) =960294a(14) =3126408a(15) =10268688a(16) =33989388a(17) =113277582a(18) =379833906a(19) =1280618784a(20) =4339003044a(21) =14767407522a(22) =50464951224a(23) =173099580168a(24) =595786322292