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arXiv · 1903.11225

Two Single-Reference Approaches to Singlet Biradicaloid Problems: Complex, Restricted Orbitals and Approximate Spin-Projection Combined With Regularized Orbital-Optimized Møller-Plesset Perturbation Theory

Joonho Lee·Martin Head-Gordon
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Abstract

We present a comprehensive study of two single-reference approaches to singlet biradicaloids. These two approaches are based on the recently developed regularized orbital-optimized Møller-Plesset method ($κ$-OOMP2). The first approach is to combine the Yamaguchi's approximate projection (AP) scheme and $κ$-OOMP2 with unrestricted (U) orbitals ($κ$-UOOMP2). By capturing only essential symmetry breaking, $κ$-UOOMP2 can serve as a suitable basis for AP. The second approach is $κ$-OOMP2 with complex, restricted (cR) orbitals ($κ$-cROOMP2). Though its applicability is more limited due to the comparative rarity of cR solutions, $κ$-cROOMP2 offers a simple framework for describing singlet biradicaloids with complex polarization while removing artificial spatial symmetry breaking. We compare the scope of these two methods with numerical studies. We show that AP+$κ$-UOOMP2 and $κ$-cROOMP2 can perform similarly well in the TS12 set, a data set that includes 12 data points for triplet-singlet gaps of several atoms and diatomic molecules with a triplet ground state. This was also found to be true for the barrier height of a reaction involving attack on a cysteine ion by a singlet oxygen molecule. However, we also demonstrate that in highly symmetric systems like $\text{C}_{30}$ ($\text{D}_{5h}$) $κ$-cROOMP2 is more suitable as it conserves spatial symmetry. Lastly, we present an organic biradicaloid that does not have a $κ$-cROOMP2 solution in which case only AP+$κ$-UOOMP2 is applicable. We recommend $κ$-cROOMP2 whenever complex polarization is essential and AP+$κ$-UOOMP2 for biradicaloids without essential complex polarization but with essential spin-polarization.

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Joonho Lee, Martin Head-Gordon. 2019-06-27. Two Single-Reference Approaches to Singlet Biradicaloid Problems: Complex, Restricted Orbitals and Approximate Spin-Projection Combined With Regularized Orbital-Optimized Møller-Plesset Perturbation Theory. https://doi.org/10.1063/1.5097613

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