Attaining high accuracy for charge-transfer excitations in non-covalent complexes at second-order perturbation cost: the importance of state-specific self-consistency

TL;DR

This paper introduces an extension of self-consistent perturbation methods that accurately predict charge-transfer excitation energies in non-covalent complexes at a computational cost comparable to lower-level methods.

Contribution

The authors develop and assess a self-consistent perturbation approach, OBMP2, for excited states, achieving high accuracy for charge-transfer states with reduced computational scaling.

Findings

OBMP2 yields small errors compared to high-level coupled cluster methods.

O2BMP2 can reach CC3 accuracy with errors less than 0.1 eV.

The methods outperform other $N^5$ scaling approaches like CC2 and ADC(2).

Abstract

Intermolecular charge-transfer (xCT) excited states important for various practical applications are challenging for many standard computational methods. It is highly desirable to have an affordable method that can treat xCT states accurately. In the present work, we extend our self-consistent perturbation methods, named one-body second-order M{\o}ller-Plesset (OBMP2) and its spin-opposite scaling variant, for excited states without additional costs to the ground state. We then assessed their performance for the prediction of xCT excitation energies. Thanks to self-consistency, our methods yield small errors relative to high-level coupled cluster methods and outperform other same scaling ($N^5$) methods like CC2 and ADC(2). In particular, the spin-opposite scaling variant (O2BMP2), whose scaling can be reduced to $N^4$, can even reach the accuracy of CC3 ($N^7$) with errors less than 0.1 eV. This method is thus highly promising for treating xCT states in large compounds vital for applications.