<h3>Title</h3>
<h3>Using response theory based methods to understand weak interactions in open-shell systems</h3>
<p><strong><span style="text-decoration: underline;">Abstract</span> :</strong></p>
<p>Interaction of radicals (open-shell) with closed-shell systems occurs in various environments of practical interest such as in atmospheric, polymer, or biological chemistry. Unlike closed shell complexes, radical– closed shell complexes may give rise to interactions that can have partial covalent bond character. Here, we take a set of molecules with doublet ground state interacting with polar molecules H<sub>2</sub>O and HF [1]. These molecules span a variety of interactions such as dispersion, hydrogen bonded, as well as having partial covalent character. Wavefunction based methods are limited to small molecules due to high computational scaling (eg., N<sup>7</sup> for CCSD(T)). The performance of DFAs is inconsistent across different approximations and among different set of complexes [1].</p>
<p>Generalised Kohn Sham random phase approximation (GKS-spRPA) has been reliable for binding energies for noncovalent complexes in S22 data set with a mean absolute error of 0.64 kcal/mol [2] and has N<sup>4</sup> scaling. Now, we test the weak interactions in 8 open shell complexes using GKS-spRPA. For our test set, we report that the mean absolute error in the interaction energies is 1.83 kcal/mol with respect to CCSD(T). Role of second order exchange interactions was tested to estimate self-interaction errors due to unpaired electrons. We use approximate exchange kernel (AXK) which has been shown to outperform adiabatic connection second-order screened exchange (ACSOSEX) [3]. AXK reduces the mean absolute error in the interaction energy to 0.68 kcal/mol. We also report that the semi-local DFAs like PBE, PBE-D3, and TPSS showed mean absolute errors of 3.93 kcal/mol, 4.43 kcal/mol, and 3.15 kcal/mol respectively.</p>
<p>For certain low-gap systems, convergence of the orbital optimization step of GKS-spRPA was challenging. This required the use of larger number of grid-points making the method costly by a factor, or the use of empirical shift parameters. Thus, we use a perturbative expansion of the RPA energy functional as a function of the orbital rotation parameter and derive a perturbative correction for orbital relaxation effects. The results from this approach will be discussed for the same test set.</p>
<p><span style="text-decoration: underline;">References </span></p>
<p>[1] Peter R Tentscher and J Samuel Arey. Binding in radical-solvent binary complexes: Benchmark energies and performance of approximate methods. Journal of Chemical Theory and Computation, 9(3):1568– 1579, 2013.</p>
<p>[2] Vamsee K Voora, Sree Ganesh Balasubramani, and Filipp Furche. Variational generalized kohn-sham approach combining the random-phase-approximation and green’s-function methods. Physical Review A, 99(1), 2019.</p>
<p>[3] Jefferson E Bates and Filipp Furche. Communication: Random phase approximation renormalized many-body perturbation theory. The Journal of chemical physics, 139(17):171103, 2013.</p>