% ising_classical_ground   Ground state energy of the classical Ising model %   ising_classical_ground(B) gives the ground state energy per spin for %   the classical Ising model in transverse field if the %   field strength is B and the coefficient %   of the nearest neighbor coupling is 1.%   That is, the Hamiltonian is H= - sum_k z(k) z(k+1) + B*sum_k x(k), %   where x and z denote Pauli spin matrices.% Copyright (C) 2005  Geza Toth    E.mail: toth@alumni.nd.edu%% This program is free software; you can redistribute it and/or% modify it under the terms of the GNU General Public License% as published by the Free Software Foundation; see gpl.txt% of this subroutine package.%% This program is distributed in the hope that it will be useful,% but WITHOUT ANY WARRANTY; without even the implied warranty of% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the% GNU General Public License for more details.% % You should have received a copy of the GNU General Public License% along with this program; if not, write to the Free Software% Foundation, Inc., 51 Franklin Street, Fifth Floor, % Boston, MA  02110-1301, USA.function E0=ising_classical_ground(B)% See G. Toth, Phys. Rev. A 71, 010301 (R) (2005).if abs(B)>2,    E0=-B;else    E0=-(1+B^2/4);end %if