Computing accurate excitation energies for large systems is one of the major challenges in theoretical chemistry. Localized orbital scaling correction (LOSC), which is an exchange-correlation (XC) functional developed to eliminate the delocalization error, is applied in the linear response theory for calculating excitation energies. We show that calculations with particle-particle random phase approximation (pp-RPA) based on LOSC dramatically lead to systematically improved accuracy in pp-RPA, while calculations with time-dependent density functional theoty (TDDFT) based on LOSC overestimate the excitation energies under the adiabatic approximation. In TDDFT the calculation requires the XC kernel, which is the second derivative of XC functional and frequency-independent under the adiabatic approximation. While in pp-RPA, the XC kernel is not required, the accuracy is closely related to the generalized Kohn-Sham orbitals and eigenvalues. The improved accuracy of pp-RPA indicate that LOSC improves orbitals and orbital energies, and the unsatisfactory results with TDDFT point to two possible remaining errors: the XC kernel and the adiabatic approximation. This highlights the importance of considering second derivative when designing functionals and the importance of a frequency-dependent linear response formalism beyond the adiabatic approximation.

Excitation energies from linear response theories with localized orbitals scaling correction in density functional theory

2019 Fall National Meeting

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