Fix: Fix missing EXX contribution in RT-TDDFT evolution steps

[AsTonyshment]

Fix

Fix #7068.

Description

When running RT-TDDFT with hybrid functionals (EXX), a severe, unphysical energy jump (specifically in eband) occurs between the initial ground-state SCF and the first electron evolution step (istep = 1).

The root cause of this bug lies in the OperatorEXX::contributeHk function. In ABACUS, the two_level_step flag is reset to 0 at the beginning of each new ionic or evolution step. Originally, the code in contributeHk contained the following condition:

if (PARAM.inp.calculation != "nscf" && this->two_level_step != nullptr && *this->two_level_step == 0 && !this->restart) { return; }

Because RT-TDDFT uses Add_Hexx_Type::k (and thus relies on contributeHk to add the EXX matrix to the Hamiltonian), this condition erroneously triggered a return for all istep >= 1 evolution steps. As a result, the EXX contribution was completely omitted from the Hamiltonian for istep >= 1, leading to a catastrophic energy jump.

A similar issue in KSDFT BOMD (which uses OperatorEXX::contributeHR) was previously identified and fixed in PR #5403 (by Yuyang Ji @1041176461) by restricting this skip condition to the first step (this->istep == 0). However, contributeHk was overlooked during that fix.

Changes made

• Updated the return condition in OperatorEXX::contributeHk to include this->istep == 0:

if (this->istep == 0 && PARAM.inp.calculation != "nscf" && this->two_level_step != nullptr && *this->two_level_step == 0 && !this->restart) { return; }

This ensures that the two-level SCF skip logic (converging pure GGA before turning on EXX) is strictly limited to the initial SCF phase (istep = 0). For all subsequent RT-TDDFT evolution steps (istep >= 1), the EXX contribution is correctly calculated and added to the Hamiltonian without being skipped.

Testing

• Before fix, total energy jumps from -6.8862606 Ry to -5.1358612 Ry:

• After fix, total energy stays the same:

Acknowledgments

Special thanks to Xinyuan Liang @xuan112358 for her valuable help in tracking down the root cause of this bug!

[mohanchen]

Nice fix!