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Before exiting, potfit will print out an error report which contains information about the errors generated by the potential.
Here is what it might look like:
###### error report ###### total error sum = 119.010627 (3173 contributions including 3108 forces, 8 energies and 48 stresses) sum of force-errors = 1.109556e+02 ( 93.232% - av: 3.570000e-02) sum of energy-errors = 8.860764e-01 ( 0.745% ) sum of stress-errors = 7.168961e+00 ( 6.024% ) root-mean-square deviations: force 7.098293e-02 (70.982927 meV/A) energy 3.276477e-02 (32.764770 meV) stress 7.729252e-02 (0.482475 MPa) Runtime: 0 hours, 3 minutes and 18 seconds. 249392 force calculations, each took 0.000794 seconds
This line contains the total error sum as an dimensionless absolute number as well as the number of summands for the sum. Each atom contributes three forces, one for each cartesion direction, and each configuration contributes six stress tensor components.
For each quantity like forces, energies and stresses the individual sum is listed together with the percentage it makes up in the total error sum. This line:
sum of force-errors = 1.109556e+02 ( 93.232% - av: 3.570000e-02)
means that about 110.9 of the 119 error value is coming from forces, which makes it 93.232%. On average each atom has a contribution of 0.0357 to the error sum (which is 110 / 3108). The stress line is only present if stress computation is enabled in the potfit binary.
These lines should give a rough estimate how good or bad the force, energy and stress values for the final potential are. For a potential which can reproduce the reference values exactly these values will all be zero. More details can be found here: https://en.wikipedia.org/wiki/Root-mean-square_deviation.