> Besides DIIS and CONJG, does Firefly include other solvers for
> the analytic Hessian calculation? If so, which cost less memory
> than CONJG?
Sorry, there are only two (DIIS and CONJG) solvers available.
>Can the analytic Hessian calculation be applied to B3LYP?
It is impossible to do so with current Firefly versions.
This feature will most likely be implemented in Firefly versions 8.1.x