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@ -178,7 +178,7 @@ Therefore, the transformed Hamiltonian
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depends on a flow parameter $s$.
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The resulting Hamiltonian possess up to $N$-body operators with $N$ the number of particle.
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\begin{equation}
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\bH(s) = E_0(s) + \bF(s) + \bV(s) + \bW(s) + \dots
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\bH(s) = E_0(s) + \bF(s) + \titou{\bV(s)} + \bW(s) + \dots
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\end{equation}
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In the following, we will truncate every contribution superior to two-body operators.
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We can easily derive an evolution equation for this Hamiltonian by taking the derivative of $\bH(s)$. This gives
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@ -206,7 +206,7 @@ In this work, we will use Wegner's canonical generator which is defined as
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\boldsymbol{\eta}^\text{W}(s) = \comm{\bH^\text{d}(s)}{\bH(s)} = \comm{\bH^\text{d}(s)}{\bH^\text{od}(s)}.
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\end{equation}
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This generator has the advantage of defining a true renormalisation scheme, \ie the coupling coefficients with the highest energy determinants are removed first.
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One of the flaws of this generator is that it generates a stiff set of ODE which is difficult to solve numerically.
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One of the flaws of this generator is that it generates a \titou{stiff} set of ODE which is difficult to solve numerically.
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However, here we consider analytical perturbative expressions so we will not be affected by this problem.
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%=================================================================%
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@ -228,7 +228,7 @@ In this case, we want to decouple the reference determinant from every singly an
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Hence, we define the off-diagonal Hamiltonian as
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\begin{equation}
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\label{eq:hamiltonianOffDiagonal}
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\hH^{\text{od}}(s) = \sum_{ia} f_i^a(s)\no{a}{i} + \frac{1}{4} \sum_{ijab}v(s)_{ij}^{ab}\no{ab}{ij}.
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\hH^{\text{od}}(s) = \sum_{ia} f_i^a(s)\no{a}{i} + \frac{1}{4} \sum_{ijab}\titou{v(s)_{ij}^{ab}}\no{ab}{ij}.
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\end{equation}
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Note that each coefficients depend on $s$.
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@ -237,7 +237,7 @@ The perturbative parameter $\la$ is such that
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\bH(0) = E_0(0) + F(0) + \la V(0)
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\end{equation}
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In addition, we know the following initial conditions.
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We use the HF basis set of the reference such that $F^{\text{od}}(0) = 0$ and $F^{\mathrm{d}}(0)=\delta_{pq}\epsilon_p$
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We use the HF basis set of the reference such that $F^{\text{od}}(0) = 0$ and \titou{$F^{\mathrm{d}}(0)=\delta_{pq}\epsilon_p$}
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Therefore, we have
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\begin{align}
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\bH^\text{d}(0)&=E_0(0) + F^{\mathrm{d}}(0) + \la V^{\mathrm{d}}(0) & \bH^\text{od}(0)&= \la V^{\mathrm{od}}(0)
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@ -340,7 +340,7 @@ To compute the second order contribution to the Hamiltonian coefficients, we fir
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The expressions for the first commutator are computed analogously to the one of the previous subsection.
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We focus on deriving expressions for the second term.
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The one-body part of $\bH^{\text{od},(1)}(s)$ is equal to zero so two of the four terms contributing to the one-body part of $\comm{\bH^{\text{d},(1)}(s)}{\bH^{\text{od},(1)}(s)}$ are zero.
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In addition, the term $\comm{A_1}{B_2}$ is equal to zero as well because the coefficients $A_{1,i}^a$ are zero (see expression in Appendix).
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In addition, the term $\comm{A_1}{B_2}$ is equal to zero as well because the coefficients \titou{$A_{1,i}^a$} are zero (see expression in Appendix).
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So we have
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\begin{align}
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\eta_a^{i,(2)}(s) &= \comm{\bH_1^{\text{d},(0)}(0)}{\bH_1^{\text{od},(2)}(s)}_a^i + \comm{\bH_2^{\text{d},(1)}(s)}{\bH_2^{\text{od},(1)}(s)}_a^i \\
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@ -366,7 +366,7 @@ In addition, the one-body hamiltonian has no first order contribution so
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\end{align}
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After integration, using the initial condition $E_0^{(2)}(0)=0$, we obtain
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\begin{equation}
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E_0^{(2)}(s) = \frac{1}{4} \sum_{i j} \sum_{a b} \frac{\Delta_{ab}^{ij}}{\aeri{ij}{ab}}\left(1-e^{-2s (\Delta_{ab}^{ij})^2}\right)
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E_0^{(2)}(s) = \frac{1}{4} \sum_{i j} \sum_{a b} \titou{\frac{\Delta_{ab}^{ij}}{\aeri{ij}{ab}}}\left(1-e^{-2s (\Delta_{ab}^{ij})^2}\right)
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\end{equation}
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%=================================================================%
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@ -417,7 +417,7 @@ which gives the following conditions
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\subsection{Zeroth order Hamiltonian}
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%%%%%%%%%%%%%%%%%%%%%%
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The zero-th order commutator of the Wegner generator therefore gives
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The zeroth-order commutator of the Wegner generator therefore gives
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\begin{equation}
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\bEta{0} = \comm{\bHd{0}}{\bHod{0}} = \bO
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\end{equation}
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@ -456,7 +456,7 @@ Now turning to the first-order contribution to the MBPT matrix, we start by comp
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\dv{\bV{}{(1),\dagger}}{s} &= 2 \bC{\text{d}}{(0)}\bV{}{(1),\dagger}\bF^{(0)} - \bV{}{(1),\dagger}(\bF^{(0)})^2 - (\bC{\text{d}}{(0)})^2\bV{}{(1),\dagger} \\
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\dv{\bC{}{(1)}}{s} &= 2 \bC{\text{d}}{(0)}\bC{\text{od}}{(1)}\bC{\text{d}}{(0)}- (\bC{\text{d}}{(0)})^2\bC{\text{od}}{(1)} - \bC{\text{od}}{(1)}(\bC{\text{d}}{(0)})^2
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\end{align}
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The two last equations can be solved differently depending on the form of $\bF$ and $\bC{}{}$.
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The last two equations can be solved differently depending on the form of $\bF$ and $\bC{}{}$.
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\subsubsection*{Diagonal $\bC{}{(0)}$}
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In the following, upper case indices correspond to the 2h1p and 2p1h sectors while lower case indices correspond to the 1h and 1p sectors. Also the $\Delta\eps_R$ corresponds to the diagonal elements of the 2h1p and 2p1h sectors.
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