As MOPAC evolves, the meaning of LET is changing.

Now LET means essentially "I know what I'm doing, override safety checks".

Currently, LET has the following meanings:

1.In a FORCE calculation, it means that the supplied geometry is to be used, even if the gradients are large.
2. In a geometry optimization, the specified GNORM is to be used, even if it is less than 0.01.
3.In a POLAR calculation, the molecule is to be orientated along its principal moments of inertia before the calculation starts. LET will prevent this step being done.
4.In a EF calculation, allow the ΔHf to rise. Obviously, the ΔHf should decrease on every step, but sometimes, particularly with very flat potential energy surfaces, motion in response to the gradients might not result in a decrease in energy.  By using LET, small increases in ΔHf are allowed.  This will allow the gradient minimum to be reached.  This is a well-defined point, whereas the minimum in energy is not, so in this case the use of LET is justified.  Note that the use of LET will not result in motion to a different minimum.
5. When comparing protein geometries, it means that hydrogen atoms that would otherwise not be paired up, because they are on different residues, are in fact paired up.