Are Accelerated and Enhanced Wave Function Methods Accurate to Compute Static Linear and Nonlinear Optical Properties?

Key
components of organic-based electro-optic devices are challenging
to design or optimize because they exhibit nonlinear optical responses,
which are difficult to model or rationalize. Computational chemistry
furnishes the tools to investigate extensive collections of molecules
in the quest for target compounds. Among the electronic structure
methods that provide static nonlinear optical properties (SNLOPs),
density functional approximations (DFAs) are often preferred because
of their low cost/accuracy ratio. However, the accuracy of the SNLOPs
critically depends on the amount of exact exchange and electron correlation
included in the DFA, precluding the reliable calculation of many molecular
systems. In this scenario, wave function methods such as MP2, CCSD,
and CCSD­(T) constitute a reliable alternative to compute SNLOPs. Unfortunately,
the computational cost of these methods significantly restricts the
size of molecules to study, a limitation that hampers the identification
of molecules with significant nonlinear optical responses. This paper
analyzes various flavors and alternatives to MP2, CCSD, and CCSD­(T)
methods that either drastically reduce the computational cost or improve
their performance but were scarcely and unsystematically employed
to compute SNLOPs. In particular, we have tested RI-MP2, RIJK-MP2,
RIJCOSX-MP2 (with GridX2 and GridX4 setups), LMP2, SCS-MP2, SOS-MP2,
DLPNO-MP2, LNO-CCSD, LNO-CCSD­(T), DLPNO-CCSD, DLPNO-CCSD­(T0), and
DLPNO-CCSD­(T1). Our results indicate that all these methods can be
safely employed to calculate the dipole moment and the polarizability
with average relative errors below 5% with respect to CCSD­(T). On
the other hand, the calculation of higher-order properties represents
a challenge for LNO and DLPNO methods, which present severe numerical
instabilities in computing the single-point field-dependent energies.
RI-MP2, RIJK-MP2, or RIJCOSX-MP2 are cost-effective methods to compute
first and second hyperpolarizabilities with a marginal average error
with respect to canonical MP2 (up to 5% for β and up to 11%
for γ). More accurate hyperpolarizabilities can be obtained
with DLPNO-CCSD­(T1); however, this method cannot be employed to obtain
reliable second hyperpolarizabilities. These results open the way
to obtain accurate nonlinear optical properties at a computational
cost that can compete with current DFAs.