Low temperature energy function for square lattice.

Terms

a(0) =2a(1) =0a(2) =-8a(3) =-24a(4) =-72a(5) =-240a(6) =-896a(7) =-3640a(8) =-15688a(9) =-70512a(10) =-326968a(11) =-1553288a(12) =-7523520a(13) =-37026704a(14) =-184677536a(15) =-931655064a(16) =-4746324296a(17) =-24387839056a(18) =-126257024696a(19) =-658011767016