If the approximated Hessian matrix is too poor, the trust radius can go too small and RFO step scaling factor can go too large, and computational error can become non-negligible to compute actual/expected energy change ratio.
To prevent this diversion, RFO step scaling factor might have to been capped at a certain upper limit and BFGS process might have to be aborted if the factor exceeds the limit.
If the approximated Hessian matrix is too poor, the trust radius can go too small and RFO step scaling factor can go too large, and computational error can become non-negligible to compute actual/expected energy change ratio.
To prevent this diversion, RFO step scaling factor might have to been capped at a certain upper limit and BFGS process might have to be aborted if the factor exceeds the limit.