Mike & list, Where can I find a reasonable model for my delr terms, if I do assume that the bond angles are changing/there is no isotropic expansion? Is there a description somewhere in the archives that I could look at? Creating different sigma squared parameters for each path did help the R- Factor somewhat, but not the reduced chi. Here's some data: Fit with seperate sigma's: Independent points = 14.312500000 Number of variables = 6.000000000 Chi-square = 5778.957783654 Reduced Chi-square = 695.212966455 R-factor = 0.029041008 Measurement uncertainty (k) = 0.000181935 Measurement uncertainty (R) = 0.004860095 Number of data sets = 1.000000000 Guess parameters +/- uncertainties (initial guess): e0_1 = -4.0347670 +/- 2.9998980 (guessed as -4.072009 (1.919404)) ss = 0.0042860 +/- 0.0005990 (guessed as 0.004636 (0.000411)) e0 = 1.6770760 +/- 1.3230290 (guessed as 1.765958 (1.154388)) alpha = 0.0009180 +/- 0.0034050 (guessed as 0.001090 (0.002843)) ss2 = 0.0048160 +/- 0.0006910 (0.0000) ss3 = 0.0225310 +/- 0.0657250 (0.0000) Def parameters: delr_1 = 0.0036440 Set parameters: amp = 0.7 Correlations between variables: e0_1 and alpha --> 0.8542 e0 and alpha --> 0.8470 e0_1 and e0 --> 0.7407 e0_1 and ss3 --> 0.7060 alpha and ss3 --> 0.4715 ss2 and ss3 --> -0.4214 e0_1 and ss2 --> -0.3892 e0 and ss3 --> 0.3772 alpha and ss2 --> -0.2680 All other correlations are below 0.25 Fit without (original): Independent points = 14.312500000 Number of variables = 4.000000000 Chi-square = 6562.747150128 Reduced Chi-square = 636.387602437 R-factor = 0.032979787 Measurement uncertainty (k) = 0.000181935 Measurement uncertainty (R) = 0.004860095 Number of data sets = 1.000000000 Guess parameters +/- uncertainties (initial guess): e0_1 = -4.0720130 +/- 1.9194050 (guessed as -4.072009 (1.919404)) ss = 0.0046360 +/- 0.0004110 (guessed as 0.004636 (0.000411)) e0 = 1.7659560 +/- 1.1543880 (guessed as 1.765958 (1.154388)) alpha = 0.0010900 +/- 0.0028430 (guessed as 0.001090 (0.002843)) Def parameters: delr_1 = 0.0043250 Set parameters: amp = 0.7 Correlations between variables: e0_1 and alpha --> 0.8416 e0 and alpha --> 0.8045 e0_1 and e0 --> 0.7141 All other correlations are below 0.25