tweak output format

                          nx_pppm_6,ny_pppm_6,nz_pppm_6);

      mesg += fmt::format("  Dispersion stencil order = {}\n",order_6);

      mesg += fmt::format("  Dispersion estimated absolute RMS force accuracy "

                          "= {}\n",acc);

                          "= {:.8g}\n",acc);

      mesg += fmt::format("  Dispersion estimated relative force accuracy "

                          "= {}\n",acc/two_charge_force);

                          "= {:.8g}\n",acc/two_charge_force);

      mesg += "  using " LMP_FFT_PREC " precision " LMP_FFT_LIB "\n";

      mesg += fmt::format("  3d grid and FFT values/proc = {} {}\n",

                          ngrid_max, nfft_both_max);