CARTESIAN COORDINATES READ IN, AND CALCULATION...
If Cartesian coordinates are read in, but the calculation is to be carried out using internal coordinates, then either all possible geometric variables must be optimized, or none can be optimized. If only some are marked for optimization then ambiguity exists. For example, if the “X” coordinate of atom 6 is marked for optimization, but the “Y” is not, then when the conversion to internal coordinates takes place, the first coordinate becomes a bond-length, and the second an angle. These bear no relationship to the “X” or “Y” coordinates. This is a fatal error.
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