> BREAK: Singular matrix in Invert_RtR
> Anyone knows the meaning of that message.
The message means that your fit failed. The determination of new
parameters failed because the system of equations became invalid.
There are several possible reasons for this, but the most probable ones
are that your fit parameters are mutually dependent, or that at least
one of them just doesn't have any effect on the fit's quality. That's
all that can be said without seeing the actual commands and data you used.