Beyond TD-DFT: Assessing the Bethe–Salpeter Equation within the GW Approximation for Absorption Properties

Time-dependent density functional
theory (TD-DFT) has
been the
“go-to” computational approach for predicting optical
absorption properties, balancing computational efficiency and reasonable
accuracy. However, the Bethe–Salpeter equation within the GW
approximation (GW/BSE) is rapidly emerging as a powerful alternative
to overcoming key limitations of TD-DFT. This letter presents a comprehensive
evaluation of one-photon and two-photon absorption (1PA and 2PA) properties
across a chemically diverse set of fluorophores, comparing TD-DFT
with the eigenvalue self-consistent GW (evGW/BSE) and single-shot
G0W0/BSE approaches, within the resolution-of-identity
approximation, relative to resolution-of-identity second-order approximate
coupled-cluster (RI-CC2). While TD-DFT maintains its dominance for
1PA, both evGW/BSE and G0W0/BSE exhibit superiority
for 2PA, offering lower absolute errors and stronger agreement with
qualitative trends. A persistent challenge in TD-DFT is the trade-off
between quantitative accuracy and capturing structure–property
trends, with no single functional reliably achieving both. In contrast,
the tested GW/BSE approaches overcome this limitation, offering reliable
values alongside a robust linear correlation. These findings further
demonstrate evGW/BSE and G0W0/BSE as promising
frameworks for modeling optical properties including 2PA.