>1. I have added $CPHF SOLVER=DIIS $END in the input file. The analytic Hessian calculation is still proceeding when MAXIMUM RESPONSE ERROR is down to E-6. However, without SOLVER=DIIS, the calculation can normally terminate and complete at MAXIMUM RESPONSE ERROR down to E-6. Why?
Because different solvers use different convergence criteria.
>2. How can I restart the analytic Hessian calculation?
Sorry, it's impossible.