X-ray structures frequently have errors that can be corrected by a constrained geometry optimization using keyword GEO_REF. In this example, the 1CBN X-ray structure is improved using PM6. Two data sets are needed. One data set, "Crambin_X-ray_geometry.mop" holds the X-ray geometry. This is the reference that will be used by the constrained optimization. The other data set, "Constrained_Crambin_1CBN_optimization.mop" holds the geometry that will be optimized. In this case, the geometry is identical to the X-ray structure.
Reference data set, data set, and final ARC file.