In a stretched hydrogen molecule the two electrons that are paired at
equilibrium forming a bond become un-paired and localized on the individual H
atoms. In singlet diradicals or doublet triradicals such a weak paring exists
even at equilibrium. At a single-determinant SCF level of the theory the
valence electrons of a singlet system like H${}_{2}$ remain perfectly paired, and
one needs to include non-dynamical correlation to decouple the bond electron
pair, giving rise to a population of effectively-unpaired (“odd”,
radicalized) electrons.^{900, 99, 862}
When the static correlation is strong, these electrons remain mostly unpaired
and can be described as being localized on individual atoms.

These phenomena can be properly described within wave-function formalism. Within DFT, these effects can be described by broken-symmetry approach or by using SF-TDDFT (see Section 7.3.1). Below we describe how to derive this sort of information from pure DFT description of such low-spin open-shell systems without relying on spin-contaminated solutions.

The first-order reduced density matrix (1-RDM) corresponding to a
single-determinant wave function (*e.g.*, SCF or Kohn-Sham DFT) is idempotent:

$\begin{array}{cc}\hfill {\rho}_{\sigma}({\mathbf{r}}_{1})& ={\displaystyle \int {\gamma}_{\sigma}^{\text{SCF}}(1;2){\gamma}_{\sigma}^{\text{SCF}}(2;1)\mathit{d}{\mathbf{r}}_{2}}\hfill \\ \hfill {\gamma}_{\sigma}^{\text{SCF}}(1;2)& ={\displaystyle \sum _{i}^{\text{occ}}}{\psi}_{i\sigma}^{\mathrm{KS}}(1){\psi}_{i\sigma}^{\mathrm{KS}}(2),\hfill \end{array}$ | (11.126) |

where ${\rho}_{\sigma}(1)$ is the electron density of spin $\sigma $ at position ${\mathbf{r}}_{1}$, and ${\gamma}_{\sigma}^{\text{SCF}}$ is the spin-resolved 1-RDM of a single Slater determinant. The cross product ${\gamma}_{\sigma}^{\text{SCF}}(1;2){\gamma}_{\sigma}^{\text{SCF}}(2;1)$ reflects the Hartree-Fock exchange (or Kohn-Sham exact-exchange) governed by the HF exchange hole:

$\begin{array}{cc}\hfill {\gamma}_{\sigma}^{\text{SCF}}(1;2){\gamma}_{\sigma}^{\text{SCF}}(2;1)& ={\rho}_{\alpha}(1){h}_{\mathrm{X}\sigma \sigma}(1,2)\hfill \\ \hfill {\displaystyle \int {h}_{\text{X}\sigma \sigma}(1,2)\mathit{d}{\mathbf{r}}_{2}}& =1.\hfill \end{array}$ | (11.127) |

When 1-RDM includes electron correlation, it becomes non-idempotent:

$${D}_{\sigma}(1)\equiv {\rho}_{\sigma}(1)-\int {\gamma}_{\sigma}(1;2){\gamma}_{\sigma}(2;1)\mathit{d}{\mathbf{r}}_{2}\ge 0.$$ | (11.128) |

The function ${D}_{\sigma}(1)$ measures the deviation from idempotency of the
correlated 1-RDM and yields the density of effectively-unpaired (odd) electrons
of spin $\sigma $ at point ${\mathbf{r}}_{1}$.^{900, 755}
The formation of effectively-unpaired electrons in singlet systems is therefore
exclusively a correlation based phenomenon. Summing ${D}_{\sigma}(1)$ over the
spin components gives the total density of odd electrons, and integrating the
latter over space gives the mean total number of odd electrons ${\overline{N}}_{u}$:

$${D}_{u}(1)=2\sum _{\sigma}{D}_{\sigma}(1)d{\mathbf{r}}_{1},{\overline{N}}_{u}=\int {D}_{u}(1)\mathit{d}{\mathbf{r}}_{1}.$$ | (11.129) |

The appearance of a factor of 2 in Eq. (11.129) above is required
for reasons discussed in Ref. 755. In Kohn-Sham DFT, the
SCF 1-RDM is always idempotent which impedes the analysis of odd electron
formation at that level of the theory. Ref. 752 has
proposed a remedy to this situation. It was noted that the correlated 1-RDM
cross product entering Eq. (11.128) reflects an effective
exchange, also known as cumulant exchange.^{99} The KS
exact-exchange hole is itself artificially too delocalized. However, the total
exchange-correlation interaction in a finite system with strong left-right
(*i.e.*, static) correlation is normally fairly localized, largely confined
within a region of roughly atomic size.^{72} The effective exchange
described with the correlated 1-RDM cross product should be fairly localized as
well. With this in mind, the following form of the correlated 1-RDM cross
product was proposed:^{752}

$${\gamma}_{\sigma}(1;2){\gamma}_{\sigma}(2;1)={\rho}_{\sigma}(1){\overline{h}}_{\text{X}\sigma \sigma}^{\text{eff}}(1,2).$$ | (11.130) |

The function ${\overline{h}}_{\text{X}\sigma \sigma}^{\text{eff}}(1;2)$ is a model
DFT exchange hole of Becke-Roussel (BR) form used in Becke’s B05
method.^{61} The latter describes left-right static correlation
effects in terms of certain effective exchange-correlation
hole.^{61} The extra delocalization of the HF exchange hole alone
is compensated by certain physically motivated real-space corrections to
it:^{61}

$${\overline{h}}_{\text{XC}\alpha \alpha}(1,2)={\overline{h}}_{\text{X}\alpha \alpha}^{\text{eff}}(1,2)+{f}_{\mathrm{c}}(1){\overline{h}}_{\text{X}\beta \beta}^{\text{eff}}(1,2).$$ | (11.131) |

The BR exchange hole ${\overline{h}}_{\text{X}\sigma \sigma}^{\text{eff}}$ is used in B05 as an auxiliary function, such that the potential from the relaxed BR hole equals that of the exact-exchange hole. This results in relaxed normalization of the auxiliary BR hole less than or equal to unity:

$$\int {\overline{h}}_{\text{X}\sigma \sigma}^{\text{eff}}(1;2)\mathit{d}{\mathbf{r}}_{2}={N}_{X\sigma}^{\text{eff}}(1)\le 1.$$ | (11.132) |

The expression of the relaxed normalization ${N}_{\text{X}\sigma}^{\text{eff}}(\mathbf{r})$ is
quite complicated, but it is possible to represent it in closed analytic
form.^{754, 753} The smaller the relaxed normalization
${N}_{X\alpha}^{\mathrm{eff}}(1)$, the more delocalized
the corresponding exact-exchange hole.^{61} The $\alpha -\alpha $
exchange hole is further deepened by a fraction of the $\beta -\beta $
exchange hole,
${f}_{\mathrm{c}}(1){\overline{h}}_{\mathrm{X}\beta \beta}^{\mathrm{eff}}(1,2)$,
which gives rise to left-right static correlation. The local correlation factor
${f}_{\mathrm{c}}$ in Eq.(11.131) governs this deepening and
hence the strength of the static correlation at each point:^{61}

$${f}_{\text{c}}(\mathbf{r})=\mathrm{min}({f}_{\alpha}(\mathbf{r}),{f}_{\beta}(\mathbf{r}),1)$$ | (11.133a) | ||

$$0\le {f}_{\mathrm{c}}(\mathbf{r})\le 1$$ | (11.133b) | ||

$${f}_{\alpha}(\mathbf{r})=\frac{1-{N}_{\text{X}\alpha}^{\text{eff}}(\mathbf{r})}{{N}_{\text{X}\beta}^{\text{eff}}(\mathbf{r})}.$$ | (11.133c) |

Using Eqs. (11.133), (11.129), and (11.130), the density of odd electrons becomes:

$\begin{array}{cc}\hfill {D}_{\alpha}(1)& ={\rho}_{\alpha}(1)(1-{N}_{X\alpha}^{\text{eff}}(1))\hfill \\ & ={\rho}_{\alpha}(1){f}_{\mathrm{c}}(1){N}_{X\beta}^{\text{eff}}(1).\hfill \end{array}$ | (11.134) |

The final formulas for the spin-summed odd electron density and the total mean number of odd electrons read:

$\begin{array}{cc}\hfill {D}_{u}(1)& =4{a}_{\text{nd}}^{\text{op}}{f}_{\text{c}}(1)\left[{\rho}_{\alpha}(1){N}_{X\beta}^{\mathrm{eff}}(1)+{\rho}_{\beta}(1){N}_{X\alpha}^{\mathrm{eff}}(1)\right]\hfill \\ \hfill {\overline{N}}_{u}& ={\displaystyle \int {D}_{u}({\mathbf{r}}_{\mathrm{\U0001d7cf}})\mathit{d}{\mathbf{r}}_{\mathrm{\U0001d7cf}}}.\hfill \end{array}$ | (11.135) |

Here ${a}_{\text{c}}^{\text{nd-opp}}=0.526$ is the SCF-optimized linear
coefficient of the opposite-spin static correlation energy term of the B05
functional.^{61, 753}

It is informative to decompose the total mean number of odd electrons into atomic contributions. Partitioning in real space the mean total number of odd electrons ${\overline{N}}_{u}$ as a sum of atomic contributions, we obtain the atomic population of odd electrons (${F}_{A}^{\text{r}}$) as:

$${F}_{A}^{\mathrm{r}}={\int}_{{\mathrm{\Omega}}_{A}}{D}_{u}({\mathbf{r}}_{\mathrm{\U0001d7cf}})\mathit{d}{\mathbf{r}}_{1}.$$ | (11.136) |

Here ${\mathrm{\Omega}}_{A}$ is a subregion assigned to atom
$A$ in the system. To define these atomic regions in a simple way, we use
the partitioning of the grid space into atomic subgroups within Becke’s
grid-integration scheme.^{66} Since the present
method does not require symmetry breaking, singlet states are calculated in
restricted Kohn-Sham (RKS) manner even at strongly stretched bonds. This
way one avoids the destructive effects that the spin contamination has on
${F}_{A}^{\text{r}}$ and on the Kohn-Sham orbitals. The calculation of
${F}_{A}^{\text{r}}$ can be done fully self-consistently only with the RI-B05
and RI-mB05 functionals. In these cases no special keywords are needed, just
the corresponding EXCHANGE rem line for these functionals. Atomic
population of odd electron can be estimated also with any other functional in
two steps: first obtaining a converged SCF calculation with the chosen
functional, then performing one single post-SCF iteration with RI-B05 or
RI-mB05 functionals reading the guess from a preceding calculation, as shown on
the input example below:

$comment Stretched H2: example of B3LYP calculation of the atomic population of odd electrons with post-SCF RI-BM05 extra iteration. $end $molecule 0 1 H 0. 0. 0.0 H 0. 0. 1.5000 $end $rem SCF_GUESS CORE METHOD B3LYP BASIS G3LARGE PURECART 222 THRESH 14 MAX_SCF_CYCLES 80 PRINT_INPUT TRUE SCF_FINAL_PRINT 1 INCDFT FALSE XC_GRID 000128000302 SYM_IGNORE TRUE SYMMETRY FALSE SCF_CONVERGENCE 9 $end @@@ $comment Now one RI-B05 extra-iteration after B3LYP to generate the odd-electron atomic population and the correlated bond order. $end $molecule read $end $rem SCF_GUESS READ EXCHANGE BM05 PURECART 22222 BASIS G3LARGE AUX_BASIS riB05-cc-pvtz THRESH 14 PRINT_INPUT TRUE INCDFT FALSE XC_GRID 000128000302 SYM_IGNORE TRUE SYMMETRY FALSE MAX_SCF_CYCLES 0 SCF_CONVERGENCE 9 DFT_CUTOFFS 0 $end @@@ $comment Finally, a fully SCF run RI-B05 using the previous output as a guess. The following input lines are obligatory here: PURECART 22222 AUX_BASIS riB05-cc-pvtz DFT_CUTOFFS 0 $end $molecule read $end $rem SCF_GUESS READ EXCHANGE BM05 PURECART 22222 BASIS G3LARGE AUX_BASIS riB05-cc-pvtz THRESH 14 PRINT_INPUT TRUE INCDFT FALSE IPRINT 3 XC_GRID 000128000302 SYM_IGNORE TRUE SCF_FINAL_PRINT 1 SYMMETRY FALSE MAX_SCF_CYCLES 80 SCF_CONVERGENCE 8 DFT_CUTOFFS 0 $end

Once the atomic population of odd electrons is obtained, a calculation of the corresponding correlated bond order of Mayer’s type follows in the code, using certain exact relationships between ${F}_{A}^{\mathrm{r}}$, ${F}_{B}^{\mathrm{r}}$, and the correlated bond order of Mayer type ${B}_{AB}$. Both new properties are printed at the end of the output, right after the multipoles section. It is useful to compare the correlated bond order with Mayer’s SCF bond order. To print the latter, use SCF_FINAL_PRINT = 1.