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The effect of the Perdew-Zunger self-interaction correction to density functionals on the energetics of small molecules
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Image of FIG. 1.
FIG. 1.

Complex (a) and real (b) energy minimizing valence orbitals of N2 calculated with PBE+SIC. Only one spin channel is shown, orbitals of the other spin have the same shape. The top three orbitals represent the triple bond, the bottom two represent lone pairs.

Image of FIG. 2.
FIG. 2.

Mean error (ME, horizontal black lines) and mean absolute error (MAE, columns) of calculated atomization energy compared to experimental values (with zero point energy removed).48 For comparison, results obtained from calculations restricted to real orbitals are shown by striped columns. The best overall agreement is obtained with PBE+SIC/2, apart from the hybrid functionals, in particular B3LYP.

Image of FIG. 3.
FIG. 3.

Optimized orbitals corresponding to the single bond in F2 calculated with (a) PBE+SIC/2 and (b) PBE+SIC. Both spin-up and spin-down orbitals are shown. For PBE+SIC/2, the total density is not spin polarized. For PBE+SIC, the orbitals are localized to some extent on one of the atoms and the electron density is spin-polarized.

Image of FIG. 4.
FIG. 4.

Errors per electron in the total energy of the molecules (x-axis) and of the constituent atoms (y-axis). The diagonal line indicates a cancellation of errors in atomization energy. Systems in the upper left area are over bound, in the lower right area binding energy is too small. Grey points indicate the hydrogen containing molecules.

Image of FIG. 5.
FIG. 5.

Mean error (ME, horizontal lines) and mean absolute error (MAE, bars) of calculated equilibrium bond lengths compared to experimental values.50 For SIC/2 and SIC, results obtained using real orbitals are indicated by striped columns. F2 was excluded for all functionals, as it is not bound with respect to the atoms for BLYP+SIC.

Image of FIG. 6.
FIG. 6.

Bond angle deviations for H2O, NH3, and CH2. The difference of calculated and experimental50 angles H-X-H are shown for the various functionals.

Image of FIG. 7.
FIG. 7.

(a) Complex and (b) real PBE+SIC optimized valence orbitals of the planar CH3 radical. Isosurfaces of the spin-majority valence orbital densities are shown in side view (left) and top view (right). The orbital of the unpaired electron is colored. The complex orbitals have mirror symmetry with respect to the plane. The real orbital for the unpaired electron has sp 3 character and the C–H-binding orbitals are out of plane. The arrangement of the real orbitals is not favored and the ground state geometry is predicted to be pyramidal.51 Figure by Simon Klüpfel from ‘Implementation and reassessment of the Perdew-Zunger self-interaction correction’, ISBN: 978-9935-9053-8-3. Used under a Creative Commons Attribution license.

Image of FIG. 8.
FIG. 8.

Energy of the bent ethynyl radical relative to the energy of the linear geometry. The bond lengths have been optimized for each value of the bond angle. The PBE ground state geometry is bent with an angle of ≈166° in agreement with previous calculations.52 PBE+SIC and PBE+SIC/2 both favor the linear geometry in agreement with experiment.50

Image of FIG. 9.
FIG. 9.

Energy barrier and bond lengths at the saddle points for the four reactions of Table V. The deviation from reference energy, in eV, and bond lengths, in Å, is shown for the various functionals. The rectangle, 1 pm by 0.1 eV, at the origin emphasizes the different energy and length scales of the different graphs. For HFH and NH3 the results based on real orbitals are depicted by grey symbols, for H4 and H3 real and complex orbitals give identical results.

Image of FIG. 10.
FIG. 10.

Errors in total energy for the PBE-type functionals. For each of the reactions, the errors per electron for the saddle point (S), reactants (R), and separated atoms (A) is shown. The vertical difference between neighboring points corresponds to errors in atomization energy (A-R) and barrier height (R-S).

Image of FIG. 11.
FIG. 11.

Energy along a path for the H + H2 → H2 + H reaction. The x-axis shows the distance between the two fragments. The H2 bond length has been relaxed for each separation. LSD+SIC(/2) predicts an energy barrier, but also reveals an intermediate configuration that is more stable than the separated reactants. Full SIC applied to PBE gives good agreement with the CI results and corrects the LSD result enough to avoid the formation of a stable hydrogen trimer.


Generic image for table
Table I.

Deviation (in eV) of calculated atomization energy E b from experiment (with zero point energy removed48). For H2 an accurate result was used as reference.49 The energy has been calculated for the respective equilibrium geometry.

Generic image for table
Table II.

Atomization energy and equilibrium bond length of F2. In BLYP+SIC, the molecule is not stable. The binding energy decreases from the uncorrected functionals to SIC/2 to SIC. The equilibrium bond length, however, changes non-monotonously with the fraction of SIC for the GGA functionals.

Generic image for table
Table III.

Deviation (in pm) of calculated bond length d b from experimentally determined geometry.50

Generic image for table
Table IV.

Equilibrium structure of the CH3 radical. The “out of plane” angle, α, in degrees and energy difference between planar and pyramidal structure, ΔE, in meV is shown for the various SIC functionals for complex (c.) and real (r.) orbitals. The uncorrected functionals all predict the correct planar ground state.

Generic image for table
Table V.

Energy barrier for four reactions. For each saddle point, the point group, energy barrier with respect to the reactants, and bond-length are listed. The labels in bold face are used throughout the text for the saddle point.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The effect of the Perdew-Zunger self-interaction correction to density functionals on the energetics of small molecules