Thank you very much for the detailed explanation—this really helped clarify things for me! The plot and your explanation made it all very clear.

Many thanks for your detailed explanation and for checking the code — that definitely clarifies things. I really appreciate your help!

Your response is incredibly valuable—thank you so much.I suspect that the DIIS/Anderson extrapolation might have 'hit' a local minimum, so the density guess in iteration 8 happened to yield a total energy extremely close to that of the previous step, resulting in a very small ΔE.
CYC 0 ETOT(AU) -8.456120523718E+03 DETOT -8.46E+03 tst 0.00E+00 PX 1.00E+00
CYC 1 ETOT(AU) -8.390242659622E+03 DETOT 6.59E+01 tst 0.00E+00 PX 1.00E+00
CYC 2 ETOT(AU) -8.391027170997E+03 DETOT -7.85E-01 tst 3.34E-03 PX 1.13E-01
CYC 3 ETOT(AU) -8.391734802465E+03 DETOT -7.08E-01 tst 2.44E-03 PX 1.02E-01
CYC 4 ETOT(AU) -8.392064543836E+03 DETOT -3.30E-01 tst 8.38E-04 PX 4.49E-02
CYC 5 ETOT(AU) -8.392086825687E+03 DETOT -2.23E-02 tst 1.24E-04 PX 1.72E-02
CYC 6 ETOT(AU) -8.392095974656E+03 DETOT -9.15E-03 tst 2.86E-05 PX 9.66E-03
CYC 7 ETOT(AU) -8.392097542826E+03 DETOT -1.57E-03 tst 3.85E-06 PX 4.81E-03
CYC 8 ETOT(AU) -8.392097541980E+03 DETOT 8.46E-07 tst 5.41E-06 PX 3.18E-03
CYC 9 ETOT(AU) -8.392098065539E+03 DETOT -5.24E-04 tst 4.69E-06 PX 3.18E-03
While this DETOT value satisfies the default energy convergence criterion, the corresponding values suggest that the electron density had not yet fully stabilized. This aligns well with your suggestion that tightening the convergence threshold would lead to a more reliable result.
Thank you again for your guidance.