diff --git a/docs/source/mf_details.md b/docs/source/mf_details.md new file mode 100644 index 0000000000000000000000000000000000000000..7c5579b37687d642212a9f6970d8c0a6e6c8d025 --- /dev/null +++ b/docs/source/mf_details.md @@ -0,0 +1,69 @@ +# Details + +## Ground state definition +Before we proceed, we must find a way to simply represent the ground state of the system. +Assume there exists a non-interacting system $\hat{H}_\text{mf}$ whose groundstate closely resembles the interacting groundstate $| 0 \rangle$: + +$$ +| 0 \rangle \approx \Pi_{E_i \leq \mu } b_i^\dagger |\textrm{vac}\rangle, +$$ + +where $|\textrm{vac}\rangle$ is the vacuum state, $b_i^\dagger$ is the creation operator of eigenstate $i$ with energy $E_i$ of $\hat{H}_\text{mf}$ and $\mu$ is the Fermi level. +We relate the $b_i^\dagger$ particles to our original basis via a unitary transformation: + +$$ +c_i^\dagger = \sum_{k} U_{ik} b_k^\dagger. +$$ + +## Normal ordering and contractions + +Before proceeding, we define the *normal ordering* operation, $:ABC...:$, as a sorting of operators such that all the creation operators $b_i^\dagger$ below the Fermi level are to the left of the annihilation $b_i$ operators below the Fermi levels, and vice versa for the operators above the Fermi level. +Whenever the normal ordered operators acts on the ground state, it gives zero: + +$$ +:ABC...: | 0 \rangle = 0. +$$ + +Lastly, we define the *contraction* of two operators $A$ and $B$ as: + +$$ +\overline{AB} = \hat{A}\hat{B} - :AB:. +$$ + +## Expansion of the interaction term +We utilize Wick's theorem to expand the interaction term: + +$$ +c_i^\dagger c_j^\dagger c_l c_k = :c_i^\dagger c_j^\dagger c_l c_k: \\ ++ \overline{c_i^\dagger c_k} :c_j^\dagger c_l: - \overline{c_i^\dagger c_l} :c_j^\dagger c_k: - \overline{c_j^\dagger c_k} :c_i^\dagger c_l: + \overline{c_j^\dagger c_l} :c_i^\dagger c_k: + \overline{c_i^\dagger c_j^\dagger} :c_l c_k: + \overline{c_l c_k} :c_i^\dagger c_j^\dagger: \\ +- \overline{c_i^\dagger c_l} \overline{c_j^\dagger c_k} +\overline{c_i^\dagger c_k} \overline{c_j^\dagger c_l} ++\overline{c_i^\dagger c_j^\dagger} \overline{c_l c_k}. +$$ + +## Mean-field approximation to Wick terms + +We are now able to apply the mean-field approximation to the Wick terms. +Lets first apply this to the first term in the expansion: + +$$ +:c_i^\dagger c_j^\dagger c_l c_k: = \langle :c_i^\dagger c_j^\dagger c_l c_k: \rangle + \delta:c_i^\dagger c_j^\dagger c_l c_k: \approx 0, +$$ + +where the first term in the second equality is zero due to the normal ordering operation and the second term is zero since we assume deviations from the mean-field ground state are small. + +Next, we apply the mean-field approximation to the second term in the expansion: + +$$ +\overline{c_i^\dagger c_k} :c_j^\dagger c_l: \approx \\ +\langle \overline{c_i^\dagger c_k} \rangle :c_j^\dagger c_l: + \overline{c_i^\dagger c_k} \langle :c_j^\dagger c_l: \rangle - \langle \overline{c_i^\dagger c_k} \rangle \langle :c_j^\dagger c_l: \rangle = \\ +\langle c_i^\dagger c_k \rangle :c_j^\dagger c_l:. +$$ + +Repeating this for all terms in the expansion and collecting we find: + +$$ +c_i^\dagger c_j^\dagger c_l c_k \approx \\ +\langle c_i^\dagger c_k \rangle c_j^\dagger c_l - \langle c_i^\dagger c_l \rangle c_j^\dagger c_k - \langle c_j^\dagger c_k \rangle c_i^\dagger c_l + \langle c_j^\dagger c_l \rangle c_i^\dagger c_k + \langle c_i^\dagger c_j^\dagger \rangle c_l c_k + \langle c_l c_k \rangle c_i^\dagger c_j^\dagger +\\ +- \langle c_i^\dagger c_l \rangle \langle c_j^\dagger c_k \rangle + \langle c_i^\dagger c_k \rangle \langle c_j^\dagger c_l \rangle + \langle c_i^\dagger c_j^\dagger \rangle \langle c_l c_k \rangle. +$$ diff --git a/docs/source/mf_notes.md b/docs/source/mf_notes.md index 6ff0d32d5cb7799306def2699bc89efd339340a2..7a5005306f053a02d087b70d73d26f949b48e93e 100644 --- a/docs/source/mf_notes.md +++ b/docs/source/mf_notes.md @@ -1,304 +1,89 @@ -# Self-consistent mean field algorithm +# Algorithm overview -## Mean-field approximation +## Interacting problems -The full hamiltonian is: +In physics, one often encounters problems where a system of multiple particles interact with each other. +By using the second quantization notation, a general Hamiltonian of such system reads: -$$ +:::{math} +:label: hamiltonian \hat{H} = \hat{H_0} + \hat{V} = \sum_{ij} h_{ij} c^\dagger_{i} c_{j} + \frac{1}{2} \sum_{ijkl} v_{ijkl} c_i^\dagger c_j^\dagger c_l c_k -$$ - -We assume the dominant part of the ground state wavefunction comes from $\hat{H}_0$. Let's assume $b_i$ operators diagonalize the unperturbed hamiltonian: - -$$ -c_i^\dagger = \sum_{k} U_{ik} b_k^\dagger, -$$ - -such that the unperturbed groundstate wavefunction is: - -$$ -| 0 \rangle = \Pi_{E_i \leq \mu } b_i^\dagger |\textrm{vac}\rangle. -$$ - -Based on this definition, we define the normal ordering operator $:ABC...:$ such that it fulfills: - -$$ -:ABC...: | 0 \rangle = 0 -$$ - -which practically means it orders $b_i$ operators based on whether its above or below the Fermi level $\mu$. - -Under this definition of normal ordering, we define the Wick's expansion of the interaction term: - -$$ -\begin{multline} -c_i^\dagger c_j^\dagger c_l c_k = :c_i^\dagger c_j^\dagger c_l c_k: \\+ \overline{c_i^\dagger c_j^\dagger} :c_l c_k: + \overline{c_i^\dagger c_k} :c_j^\dagger c_l: - \overline{c_i^\dagger c_l} :c_j^\dagger c_k: + \overline{c_l c_k} :c_i^\dagger c_j^\dagger: - \overline{c_j^\dagger c_k} :c_i^\dagger c_l: + \overline{c_j^\dagger c_l} :c_i^\dagger c_k: \\ -\overline{c_i^\dagger c_j^\dagger} \overline{c_l c_k} - \overline{c_i^\dagger c_l} \overline{c_j^\dagger c_k} + \overline{c_i^\dagger c_k} \overline{c_j^\dagger c_l} -\end{multline} -$$ - -where the overline defines Wick's contraction: - -$$ -\overline{AB} = AB - :AB:. -$$ - -The expectation value of the interaction with respect to the $| 0 \rangle$ is: - -$$ -\langle 0 | c_i^\dagger c_j^\dagger c_l c_k | 0 \rangle = \langle 0 | \overline{c_i^\dagger c_j^\dagger} \overline{c_l c_k} - \overline{c_i^\dagger c_l} \overline{c_j^\dagger c_k} + \overline{c_i^\dagger c_k} \overline{c_j^\dagger c_l} | 0 \rangle -$$ - -where we can forget about all the normal ordered states since those give zero acting on the unperturbed groundstate. To evaluate this further, we utilize the mean-field approximation: - -$$ -A B \approx \langle A \rangle B + A \langle B \rangle - \langle A \rangle \langle B \rangle -$$ - -onto the contractions such that we get: - -$$ -\langle c_i^\dagger c_j^\dagger c_l c_k \rangle \approx \langle c_i^\dagger c_j^\dagger \rangle \langle c_l c_k \rangle + \langle c_i^\dagger c_k \rangle \langle c_j^\dagger c_l \rangle - \langle c_i^\dagger c_l \rangle \langle c_j^\dagger c_k \rangle -$$ +::: -note $\langle A B \rangle \approx \langle A \rangle \langle B \rangle$ assuming mean-field. +where $c_i^\dagger$ and $c_i$ are creation and annihilation operators respectively for fermion in state $i$. +The first term $\hat{H}_0$ is the non-interacting Hamiltonian which by itself is straightforward to solve in a single-particle basis by direct diagonalizations made easy through packages such as `kwant`. +The second term $\hat{V}$ is the interaction term between two particles. +In order to solve the interacting problem exactly, one needs to diagonalize the full Hamiltonian $\hat{H}$ in the many-particle basis which grows exponentially with the number of particles. +Such a task is often infeasible for large systems and one often needs to resort to approximations. -To consider excitations from the groundstate, we make use of the mean-field approximation defined above: - -$$ -\begin{multline} -c_i^\dagger c_j^\dagger c_l c_k \approx \\ -\langle c_i^\dagger c_j^\dagger \rangle c_l c_k + \langle c_i^\dagger c_k \rangle c_j^\dagger c_l - \langle c_i^\dagger c_l \rangle c_j^\dagger c_k + \langle c_l c_k \rangle c_i^\dagger c_j^\dagger - \langle c_j^\dagger c_k \rangle c_i^\dagger c_l + \langle c_j^\dagger c_l \rangle c_i^\dagger c_k + \\ -\langle c_i^\dagger c_j^\dagger \rangle \langle c_l c_k \rangle + \langle c_i^\dagger c_k \rangle \langle c_j^\dagger c_l \rangle - \langle c_i^\dagger c_l \rangle \langle c_j^\dagger c_k \rangle -\end{multline} -$$ - -Where we made use of the following operations: - -$$ -:c_i^\dagger c_j^\dagger c_l c_k: \approx 0 -$$ - -$$ -\overline{c_i^\dagger c_k} \overline{c_j^\dagger c_l} \approx \langle \overline{c_i^\dagger c_k} \rangle \overline{c_j^\dagger c_l} + \overline{c_i^\dagger c_k} \langle \overline{c_j^\dagger c_l} \rangle - \langle \overline{c_i^\dagger c_k} \rangle \langle \overline{c_j^\dagger c_i} \rangle = \langle c_i^\dagger c_k \rangle \overline{c_j^\dagger c_l} + \overline{c_i^\dagger c_k} \langle c_j^\dagger c_l \rangle - \langle c_i^\dagger c_k \rangle \langle c_j^\dagger c_l \rangle -$$ - -$$ -\overline{c_i^\dagger c_k} :c_j^\dagger c_l: \approx \langle \overline{c_i^\dagger c_k} \rangle :c_j^\dagger c_l: + \overline{c_i^\dagger c_k} \langle :c_j^\dagger c_l: \rangle - \langle \overline{c_i^\dagger c_k} \rangle \langle :c_j^\dagger c_l: \rangle = \langle \overline{c_i^\dagger c_k} \rangle :c_j^\dagger c_l: -$$ - - -$$ -\langle \overline{c_i^\dagger c_k} \rangle = \langle c_i^\dagger c_k - :c_i^\dagger c_k: \rangle = \langle c_i^\dagger c_k \rangle -$$ - - -Without any superconducting terms, the form simplifies to: - -$$ -\begin{multline} -c_i^\dagger c_j^\dagger c_l c_k \approx -\langle c_i^\dagger c_k \rangle c_j^\dagger c_l - \langle c_i^\dagger c_l \rangle c_j^\dagger c_k - \langle c_j^\dagger c_k \rangle c_i^\dagger c_l + \langle c_j^\dagger c_l \rangle c_i^\dagger c_k + \\ -\langle c_i^\dagger c_k \rangle \langle c_j^\dagger c_l \rangle - \langle c_i^\dagger c_l \rangle \langle c_j^\dagger c_k \rangle -\end{multline} -$$ - -## Finite size - -### Coulomb interaction - -We simplify the interaction term through the MF approximation to get: - -$$ -V = \frac{1}{2}\sum_{ijkl} v_{ijkl} c_i^{\dagger} c_j^{\dagger} c_l c_k -\approx -\frac12 \sum_{ijkl} v_{ijkl} \left[ \langle c_i^{\dagger} c_k \rangle c_j^{\dagger} c_l - \langle c_j^{\dagger} c_k \rangle c_i^{\dagger} c_l - \langle c_i^{\dagger} c_l \rangle c_j^{\dagger} c_k + \langle c_j^{\dagger} c_l \rangle c_i^{\dagger} c_k \right] -$$ -(assuming no superconductivity) - -and an additional constant part: - -$$ -V_0 = \frac{1}{2} \sum_{ijkl} v_{ijkl} \left(\langle c_j^{\dagger} c_l \rangle \langle c_i^{\dagger} c_k \rangle - \langle c_j^{\dagger} c_k \rangle \langle c_i^{\dagger} c_l \rangle \right). -$$ - -The interaction reads: - -$$ -v_{ijkl} = \iint w_{i}^*(r) w_{j}^*(r') V(r, r') w_{k}(r) w_l(r') dr dr' = \\ -\iint V(|r - r'|) w_{i}^*(r)w_{k}(r) w_{j}^*(r') w_l(r') dr dr' -$$ - -whereas $w_i$ is a wannier function on site i (and corresponding dof). Whenever one interchanges $i \to j, k \to l$, the Coulomb term is preserved $v_{ijkl} = v_{jilk}$ - -To make things more understandable, we are also going to explicitly split up position and spin indices: $i \to i \times \sigma$. In this notation, the Coulomb integral reads: - -$$ -v_{ijkl}^{\sigma_i \sigma_j \sigma_k \sigma_l} = -\iint V(|r - r'|) w_{i\times\sigma_i}^{*} (r)w_{k \times \sigma_k}(r) w_{j \times \sigma_j}^{*}(r') w_{l\times \sigma_l}(r') dr dr' \delta_{\sigma_i \sigma_k} \delta_{\sigma_{j} \sigma_l} -$$ +## Mean-field approximation -On a fine tight-binding model, we have: +In many interacting systems, there exist constant order parameters $\langle A \rangle$ that describe the phase of the system. +Here we define $\hat{A}$ as some operator and $\langle \rangle$ denotes the expectation value with respect to the ground state of the system. +Famous examples of such order parameter is the magnetization in a ferromagnet and the superconducting order parameter in a superconductor. +If we are interested in properties of the system close to the ground state, we can re-write the operator $\hat{A}$ around the order parameter: $$ -v_{ijkl}^{\sigma_i \sigma_j \sigma_k \sigma_l} = v_{ij} \delta_{ik} \delta_{jl} \delta_{\sigma_i \sigma_k} \delta_{\sigma_j \sigma_l} +\hat{A} = \langle A \rangle + \delta \hat{A}, $$ -where $v_{ij} = V(r_i, r_j)$. - -We shall re-define $i$ index to absorb spin: +where operator $\delta \hat{A}$ describes the fluctuations around the order parameter. +Let us consider an additional operator $\hat{B}$ and say we are interested in the product of the two operators $\hat{A}\hat{B}$. +If we assume that the fluctuations $\delta$ are small, we can approximate the product of operators into a sum of single operators and the product of the expectation values: $$ -\delta_{ik} \times \delta_{\sigma_{i} \sigma_{k}} \to \delta_{ik} +\hat{A}\hat{B} \approx \langle A \rangle \hat{B} + \hat{A} \langle B \rangle - \langle A \rangle \langle B \rangle $$ -in this notation the above reads: +where we neglect $\delta^2$ terms. +This approximation is known as the mean-field approximation. -$$ -v_{ijkl} = v_{ij} \delta_{ik} \delta_{jl} -$$ +## Mean-field Hamiltonian -The mean-field terms are: +We apply the mean-field approximation to the quartic interaction term in {eq}`hamiltonian`: $$ -\langle c_i^{\dagger} c_j\rangle = \langle \Psi_F|c_i^{\dagger} c_j | \Psi_F \rangle +V \approx \frac12 \sum_{ijkl} v_{ijkl} \left[ +\langle c_i^\dagger c_k \rangle c_j^\dagger c_l - \langle c_i^\dagger c_l \rangle c_j^\dagger c_k - \langle c_j^\dagger c_k \rangle c_i^\dagger c_l + \langle c_j^\dagger c_l \rangle c_i^\dagger c_k \right] $$ -whereas $|\Psi_F \rangle = \Pi_{i=0}^{N_F} b_i^\dagger |0\rangle$. To make sense of things, we need to transform between $c_i$ basis (position + internal dof basis) into the $b_i$ basis (eigenfunction of a given mean-field Hamiltonian): +where we make use of Wicks theorem to simplify the expression and neglect the superconducting pairing and constant offset terms. -$$ -c_i^\dagger = \sum_{k} U_{ik} b_k^\dagger -$$ - -where +:::{admonition} Derivation of the mean-field Hamiltonian with Wicks theorem +:class: dropdown info +```{include} mf_details.md +``` +::: +The expectation value terms $\langle c_i^\dagger c_j \rangle$ are the density matrix elements of the ground state of the system: $$ -U_{ik} = \langle{i|\psi_k} \rangle. +\rho_{ij} = \langle c_i^\dagger c_j \rangle. $$ -That gives us: +### Tight-binding grid +We project $\hat{V}$ onto a tight-binding grid: $$ -c_i^{\dagger} c_j = \sum_{k, l} U_{ik} U_{lj}^* b_k^\dagger b_{l} -$$ - -and its expectation value gives us the mean-field ... field $F_{ij}$: - -$$ -F_{ij} = \langle c_i^{\dagger} c_j\rangle = \sum_{k, l} U_{ik} U_{lj}^* \langle \Psi_F| b_k^\dagger b_{l}| \Psi_F \rangle = \sum_{k} U_{ik} U_{kj}^{*} +V_{nm} = \langle n | \hat{V} | m \rangle = \\ +\frac12 \left[ \sum_{ik} v_{inkm} F_{ik} - \sum_{jk} v_{njkm} F_{jk} - \sum_{il} v_{inml} F_{il} + \sum_{jl} v_{njml} F_{jl} \right] = \\ +-\sum_{ij} F_{ij} \left(v_{inmj} - v_{injm} \right) $$ -whereas I assumed the indices for wavefunctions $k,l$ are ordered in terms of increasing eigenvalue. We pop that into the definition of the mean-field correction $V$: +where I used the $v_{ijkl} = v_{jilk}$ symmetry from Coulomb. +For density-density interactions (like Coulomb repulsion) the interaction tensor reads: $$ -\begin{multline} -V_{nm} = \frac12 \sum_{ijkl} v_{ijkl} \langle n| \left[ \langle c_i^{\dagger} c_k \rangle c_j^{\dagger} c_l - \langle c_j^{\dagger} c_k \rangle c_i^{\dagger} c_l - \langle c_i^{\dagger} c_l \rangle c_j^{\dagger} c_k + \langle c_j^{\dagger} c_l \rangle c_i^{\dagger} c_k \right] |m\rangle = \\ - \frac12 \sum_{ijkl} v_{ijkl} \left[ +\delta_{jn}\delta_{lm} F_{ik} -\delta_{in}\delta_{lm} F_{jk} -\delta_{jn}\delta_{km} F_{il} + \delta_{in}\delta_{km} F_{jl} \right] = \\ -\frac12 \left[ \sum_{ik} v_{inkm} F_{ik} - \sum_{jk} v_{njkm} F_{jk} - \sum_{il} v_{inml} F_{il} + \sum_{jl} v_{njml} F_{jl} \right] = \\ --\sum_{ij} F_{ij} \left(v_{inmj} - v_{injm} \right) -\end{multline} +v_{ijkl} = v_{ij} \delta_{ik} \delta_{jl}, $$ -where I used the $v_{ijkl} = v_{jilk}$ symmetry from Coulomb. - -For a tight-binding grid, the mean-field correction simplifies to: +which simplifies the interaction to: $$ -\begin{multline} V_{nm} = - \sum_{ij} F_{ij} \left(v_{inmj} - v_{injm} \right) = \\ -\sum_{ij}F_{ij} v_{in} \delta_{im} \delta_{nj} + \sum_{ij}F_{ij} v_{in} \delta_{ij} \delta_{nm} = \\ -F_{mn} v_{mn} + \sum_{i} F_{ii} v_{in} \delta_{nm} -\end{multline} $$ the first term is the exchange interaction whereas the second one is the direct interaction. - -Similarly, the constant offset term reads: - -$$ -\begin{multline} -V_0 = \frac{1}{2} \sum_{ijkl} v_{ijkl} \left(F_{jl} F_{ik} - F_{jk} F_{il} \right) = \\ -\frac{1}{2} \sum_{ijkl} v_{ij} \delta_{ik} \delta_{jl} \left(F_{jl} F_{ik} - F_{jk} F_{il}\right) \\ -= \frac{1}{2} \sum_{ij} v_{ij} \left(F_{ii} F_{jj} - F_{ji} F_{ij}\right) -\end{multline} -$$ - -where we identify the first term as the exchange (mixes indices) and the right one as the direct (diagonal in indices). - -## Translational Invariance - -The above assumed a finite tight-binding model - all $nm$-indices contain the position of all atoms (among other dof). In this section tho we want to consider an infinite system with translational invariance. - -To begin with we deconstruct a general matrix $O_{nm}$ into the cell degrees of freedom ($nm$) and the position of the the cell itself ($ij$): - -$$ -O_{nm} \to O^{ij}_{nm} -$$ - -and we will Fourier transform the upper indices into k-space: - -$$ -O_{mn}(k) = \sum_{ij} O_{nm}^{ij} e^{-i k (R_i-R_j)} -$$ - -where I assumed $O$ (and thus all operators I will consider here) is local and thus diagonal in k-space. - -Now lets rewrite our main result in the previous section using our new notation: - -$$ -V_{nm}^{ij} =-F_{mn}^{ij} v_{mn}^{ij} + \sum_{r,p} F_{pp}^{rr} v_{pn}^{rj} \delta_{nm} \delta^{ij} -$$ - -Lets first consider the second (direct) term. Lets express the corresponding $F$ term in k-space: - -$$ -F_{pp}^{rr} = \int e^{i k (R_r-R_r)} F_{pp}(k) dk = \int F_{pp}(k) dk -$$ - -Notice that in the final expression, there is no $rr$ dependence and thus this term is cell-periodic. Therefore, we shall redefine it as cell electron density $\rho$: -$$ -F_{pp}^0 = F_{pp}(R = 0) = \int F_{pp}(k) dk -$$ - -Now since $\rho$ has no $r$ dependence, we can proceed with the sum: - -$$ -\sum_{r} v_{pn}^{rj} = \int v_{pn}(k) e^{ik R_j} \sum_{r} e^{-i k R_r} dk = \int v_{pn}(k) e^{ik R_j} \delta_{k, 0} dk = v_{pn}(0) -$$ - -We are finally ready to Fourier transform the main result. Invoking convolution theorem and the results above gives us: - -$$ -V_{nm}(k) = \sum_{p} F_{pp}^0 v_{pn}(0) \delta_{nm} -F_{mn}(k) \circledast v_{mn}(k) = V_n^D - F_{mn}(k) \circledast v_{mn}(k) -$$ - -which does make sense. The first term (direct) is a potential term coming from the mean-field and the second term (exchange) is purely responsible for the hopping. - -The constant offset is: -$$ -V_0 = \frac{1}{2} \sum_{r,s} \rho_r v_{rs}(0) \rho_s- \\ \frac{1}{2} tr\left[\int_{BZ} \left(F \circledast v\right)(k) F(k) dk \right] -$$ - -## Short-Range interactions - -In the case of short-range interactions, it is much more convenient to go back to real space to both store objects and perform the operations. In real space the mean-field part of the Hamiltonian reads: - -$$ -V_{nm}(\mathbf{R}) = V_n^D \delta(\mathbf{R}) - F_{mn}(\mathbf{R}) v_{mn}(\mathbf{R}) -$$ - -(the first term might need some prefactor from Fourier transformation) - -where $\mathbf{R}$ is a sequence of integers representing real-space unit cell indices. - -### Proposed Algorithm -Given an initial Hamiltonian $H_0 (R)$ and the interaction term $v(R)$ in real-space, the mean-field algorithm is the following: - -0. Start with a mean-field guess in real-space: $V(R)$. -1. Fourier transform tight-binding model and the mean-field in real space to a given k-grid: $H_0(R) + V(R) \to H_0(k) + V(k)$ -2. Diagonalize and evaluate the density matrix: $H_0(k) + V(k) \to F(k)$ -3. Fourier transform the density matrix back to real-space: $F(k) \to F(R)$ -4. Evaluate the new mean-field Hamiltonian $V(R)$ according to the equation above. -5. Evaluate self-consistency metric (could be based either on $V(R/k)$ or $F(R/k)$). Based on that, either stop or go back to 1 with a modified $V(R)$ starting guess.