diff --git a/Manuscript/rsdft-cipsi-qmc.tex b/Manuscript/rsdft-cipsi-qmc.tex index c45b08a..9b33d9e 100644 --- a/Manuscript/rsdft-cipsi-qmc.tex +++ b/Manuscript/rsdft-cipsi-qmc.tex @@ -607,31 +607,77 @@ Ref.~\onlinecite{Scemama_2015}). \end{table} In this section, we make a numerical verification that the produced -wave functions are size-consistent. We have computed the energy of the -dissocited fluorine dimer, where the two atoms are at a distance of 50~\AA. -We expect that the energy of this system is equal to twice the energy -of the fluorine atom. -The data in table~\ref{tab:size-cons} shows that the proposed scheme -provides size-consistent FN-DMC energies for all values of $\mu$. +wave functions are size-consistent for a given range-separation +parameter. +We have computed the energy of the dissocited fluorine dimer, where +the two atoms are at a distance of 50~\AA. We expect that the energy +of this system is equal to twice the energy of the fluorine atom. +The data in table~\ref{tab:size-cons} shows that it is indeed the +case, so we can conclude that the proposed scheme provides +size-consistent FN-DMC energies for all values of $\mu$. \subsection{Spin-invariance} -Closed-shell molecules usually dissociate into open-shell +Closed-shell molecules often dissociate into open-shell fragments. To get reliable atomization energies, it is important to have a theory which is of comparable quality for open-shell and -closed-shell systems. +closed-shell systems. A good test is to check that all the components +of a spin multiplet are degenerate. FCI wave functions are invariant with respect to the spin quantum number $m_s$, but the introduction of a -Jastrow factor breaks this spin-invariance if the parameters +Jastrow factor introduces spin contamination if the parameters for the same-spin electron pairs are different from those for the opposite-spin pairs.\cite{Tenno_2004} -Again, using pseudo-potentials this error is transferred in the DMC -calculation unless the determinant localization approximation is used. +Again, when pseudo-potentials are used this tiny error is transferred +in the FN-DMC energy unless the determinant localization approximation +is used. + +Within DFT, the common density functionals make a difference for +same-spin and opposite-spin interactions. As DFT is a +single-determinant theory, the functionals are designed to work in +with the highest value of $m_s$, and therefore different +values of $m_s$ lead to different energies. +So in the context of RS-DFT, the determinantal expansions will be +impacted by this spurious effect, as opposed to FCI. + +\begin{table} + \caption{FN-DMC Energies of the triplet carbon atom (BFD-VDZ) with + different values of $m_s$.} + \label{tab:spin} + \begin{ruledtabular} + \begin{tabular}{cccc} + $\mu$ & $m_s=1$ & $m_s=0$ & Spin-invariance error \\ + \hline + 0.00 & $-5.416\,8(1)$ & $-5.414\,9(1)$ & $+0.001\,9(2)$ \\ + 0.25 & $-5.417\,2(1)$ & $-5.416\,5(1)$ & $+0.000\,7(1)$ \\ + 0.50 & $-5.422\,3(1)$ & $-5.421\,4(1)$ & $+0.000\,9(2)$ \\ + 1.00 & $-5.429\,7(1)$ & $-5.429\,2(1)$ & $+0.000\,5(2)$ \\ + 2.00 & $-5.432\,1(1)$ & $-5.431\,4(1)$ & $+0.000\,7(2)$ \\ + 5.00 & $-5.431\,7(1)$ & $-5.431\,4(1)$ & $+0.000\,3(2)$ \\ + $\infty$ & $-5.431\,6(1)$ & $-5.431\,3(1)$ & $+0.000\,3(2)$ \\ + \end{tabular} + \end{ruledtabular} +\end{table} + +In this section, we investigate the impact of the spin contamination +due to the short-range density functional on the FN-DMC energy. We have +computed the energies of the carbon atom in its triplet state +with BFD pseudo-potentials and the corresponding double-zeta basis +set. The calculation was done with $m_s=1$ (3 $\uparrow$ electrons +and 1 $\downarrow$ electrons) and with $m_s=0$ (2 $\uparrow$ and 2 +$\downarrow$ electrons). + +The results are presented in table~\ref{tab:spin}. +Although using $m_s=0$ the energy is higher than with $m_s=1$, the +bias is relatively small, more than one order of magnitude smaller +than the energy gained by reducing the fixed-node error going from the single +determinant to the FCI trial wave function. The highest bias, close to +2~m\hartree, is obtained for $\mu=0$, but the bias decreases quickly +below 1~m\hartree when $\mu$ increases. As expected, with $\mu=\infty$ +there is no bias (within the error bars), and the bias is not +noticeable with $\mu=5$~bohr$^{-1}$. -To check that the RSDFT-CIPSI are spin-invariant, we compute the -FN-DMC energies of the ?? dimer with different values of the spin -quantum number $m_s$.