Highly accurate wavefunctions for two-electron systems using two parameteres

It is shown for two electron atoms that ground-state wavefunctions of the form \begin{equation} Ψ(\vec{r_{1}}, \vec{r_{2}})=ϕ(\vec{r_{1}})ϕ(\vec{r_{2}})(\cosh ar_{1}+\cosh ar_{2})(1+0.5 r_{12}e^{-b r_{12}}) \end{equation} where $\vec{r_{1}}$ and $\vec{r_{2}}$ are the coordinates of two electrons and $r_{12}=|\vec{r_{1}}-\vec{r_{2}}|$, can be made highly accurate by optimizing $a$, $b$ and $ϕ$. This is done by solving a variationally derived equation for $ϕ$ for a given $a$ and $b$ and finding $a$ and $b$ so that the expectation value of the Hamiltonian is minimum. For the set $\{a, b, ϕ\}$ the values for various quantities obtained from the above wavefunction are compared with those given by $204$-parameter wavefunction of Koga et al.[11] and are found to be matching quite accurately(within ppm) with them.

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