Journal Article10.1063/5.0227556
Efficient random phase approximation for diradicals
Reza G. Shirazi,Vladimir V. Rybkin,Michael Marthaler,Dmitry S. Golubev +3 more
TL;DR: Researchers apply the random phase approximation to diradical molecules, showing that it renormalizes model parameters and accurately predicts singlet-triplet splitting for 13 molecules, rivaling the NEVPT2 method with two orbitals and two electrons in the active space.
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Abstract: We apply the analytically solvable model of two electrons in two orbitals to diradical molecules, characterized by two unpaired electrons. The effect of doubly occupied and empty orbitals is taken into account by means of random phase approximation (RPA). We show that in the static limit, the direct RPA leads to the renormalization of the parameters of the two-orbital model. We test our model by comparing its predictions for singlet–triplet splitting with the results of several multi-reference methods for a set of thirteen molecules. We find that for this set, the static RPA results are close to those of the NEVPT2 method with two orbitals and two electrons in the active space.
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Figures
FIG. 5. p-benzyne. 
FIG. 6. 2,5-didehydropyridinium cation (DDP-1). 
FIG. 4. m-benzyne. 
FIG. 9. trimethylenemethane (TMM). 
FIG. 10. phenylnitrene (PN) 
TABLE I. Siglet-triplet splitting (kcal/mol) for several molecules. The number of orbitals in the active space for CASSCF and NEVPT2 simulations has been chosen as follows: 8 orbitals for p−benzyne, m−benzyne, o−benzyne, DDP-1, PN and PC; 9 orbitals for DDP-2; 6 orbitals for TME; 5 orbitals for CPC; and 4 orbitals for TMM. The last row shows the ratio of the singlet-triplet gap to the minimum enegry splitting between the empty and the doubly occupied orbitals ∆εmin. The last column "Error" shows the absolute value of the error (kcal/mol) relative to NEVPT2 method averaged over all ten molecules. a For this simulations single and two electron integrals for 104 orbitals obtained from CASSCF(2,2) have been used. b For this simulations single and two electron integrals for 128 orbitals obtained from CASSCF(2,2) have been used.
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