%
  function ss = Helmholtz_ss(params,data)

  udata = data.ydata;
  P = data.xdata;

  alpha_1 = params(1);
  alpha_11 = params(2);
  alpha_111 = params(3);

  uvals_data = alpha_1*P.^2 + alpha_11*P.^4 + alpha_111*P.^6;

  res = udata - uvals_data;
  ss = res'*res;