% Takes one time step of size k using the magnus midpoinr
% starting from v_0 and where the function F(v,t) gives the
% time derivatives.
function v = Magnus_mp(v,D, t , k)

if isa(D,'function_handle')
   % v = krylov(k*D(t +k/2*t),v);
%   v = expm(k*D(t +k/2))*v;
      toler = 10^(-5);
   expm_Arnoldi(-D,v,k,toler,100)
else
   %v = krylov(k*D,v);
   % v = expm(k*D)*v;
end

end