AbstractThe uncertainties in the refined parameters for a 1.5‐Å X‐ray structure of carbon‐monoxy (FeII) myoglobin are estimated by combining energy minimization with least‐squares refinement against the X‐ray data. The energy minimization, done without reference to the X‐ray data, provide perturbed structures which are used to restart conventional X‐ray refinement. The resulting refined structures have the same, or better, R‐factor and stereochemical parameters as the original X‐ray structure, but deviate from it by 0.13 Å rms for the backbone atoms and 0.31 Å rms for the sidechain atoms. Atoms interacting with a disordered sidechain, Arg 45 CD3, are observed to have larger positional uncertainties. The uncertainty in the B‐factors, within the isotropic harmonic motion approximation, is estimated to be 15%. The resulting X‐ray structures are more consistent with the energy parame