The application of Dyall Hamiltonian-based MRPT2 in high-lying electronically excited state calculations

As the MRPT2 methods based on the Dyall Hamiltonian, the MS-NEVPT2 and SDSPT2 are both robust to calculate low-lying excited states of strongly correlated systems without intruder state problem (ISP). However, their applicability to high-lying excited states remains underexplored in detail. To address this, we systematically evaluated 30 electronic states across 21 organic molecules using MS-NEVPT2 and SDSPT2. Our analysis confirms that both methods effectively mitigate the ISP and achieve reasonably vertical excitation energies when appropriate level shift parameters are applied. This comprehensive study establishes essential theoretical references for modeling high-lying excited states in molecular systems using MS-NEVPT2 and SDSPT2. • Systematic Evaluation: This study comprehensively evaluates the applicability of Dyall Hamiltonian-based MRPT2 methods (MS-NEVPT2 and SDSPT2) for calculating high-lying electronically excited states in 21 organic molecules (30 singlet states). • ISP Mitigation Strategies: Both MS-NEVPT2 and SDSPT2 are shown to mitigate the intruder-state problem (ISP) when combined with real level shift (RLS) parameters. • ISP Localization: ISP predominantly arises in three Q subspaces of ( V ̄ D ) , ( D ̄ V ) , and ( D ̄ D ) .