# Rational Energy Model chemtools.tool.globaltool.RationalGlobalTool¶

In this model, energy is approximated by a rational function of the number of electrons. In the most general form, this model can be written as:

$E^{(m,n)}\left(N\right) = \left( \frac{a_0 + a_1N + a_2{N^2} + ... + a_m{N^m}}{1 + b_1N + b_2{N^2} + ... + b_n{N^n}} \right) = \frac{\sum_{j=0}^{m} a_j N^j}{1 + \sum_{i=1}^{n} b_i N^i}$

The number of unknown parameters in this model depends on the values of $$m$$ and $$n$$. Having a set of $$m+n$$ values of $$N$$ for which the energy is known, the model can be parametrized by solving a system of linear equations. By rearranging the rational energy expression above, the equations can be written as:

$\sum_{j=0}^{m} \left(N^j\right) a_j - \sum_{i=1}^{n} \left(N^i \cdot E^{(m,n)}\left(N\right) \right) b_i = E^{(m,n)}\left(N\right)$

Having the parameters $$\{a_j\}_{j=0}^m$$ and $$\{b_i\}_{i=1}^n$$, the energy model is known, and the derivatives of the rational energy model with respect to the number of electrons at fixed external potential can be calculated.

However, in order to solve for the parameters in this model analytically, a simpler form of the rational energy model containing three parameters, $$E^{(2,1)}\left(N\right) = E\left(N\right)$$, is considered. For implementing more complex rational energy models, please refer to the general energy model.

$E\left(N\right) = E^{(2,1)}\left(N\right) = \frac{a_0 + a_1 N}{1 + b_1 N}$

Containing three parameters, $$a_0$$, $$a_1$$ and $$b_1$$, this model requires three values of $$E\left(N\right)$$ to interpolate the energy. Commonly, the energy of the system with $$N_0 - 1$$, $$N_0$$ and $$N_0 + 1$$ electrons are provided. Fitting the energy expression to the given energy values results in three equations:

$\begin{split}\begin{cases} \left(1 + b_1 \left(N_0 - 1\right)\right) & E\left(N_0-1\right) &&= a_0 + a_1 \left(N_0 - 1\right) \\ \left(1 + b_1 N_0\right) & E\left(N_0\right) &&= a_0 + a_1 N_0 \\ \left(1 + b_1 \left(N_0 + 1\right)\right) & E\left(N_0+1\right) &&= a_0 + a_1 \left(N_0 + 1\right) \\ \end{cases}\end{split}$

This allows us to solve for the three unknowns:

$\begin{split}b_1 &= -\frac{E\left(N_0 + 1\right) - 2 E\left(N_0\right) + E\left(N_0 - 1\right)} {\left(N_0 + 1\right) E\left(N_0 + 1\right) - 2 N_0 E\left(N_0\right) + \left(N_0 - 1\right) E\left(N_0 - 1\right)} \\ a_1 &= \left(1 + b_1 N_0\right) \left(E\left(N_0 + 1\right) - E\left(N_0\right)\right) + b_1 E\left(N_0 + 1\right) \\ a_0 &= - a_1 N_0 + \left(1 + b_1 N_0\right) E\left(N_0\right)\end{split}$
$\begin{split}a_0 &= \frac{E\left(N_0\right) E\left(N_0-1\right) N_{0} + E\left(N_0\right) E\left(N_0-1\right) + E\left(N_0\right) E\left(N_0+1\right) N_{0} - E\left(N_0\right) E\left(N_0+1\right) - 2 E\left(N_0-1\right) E\left(N_0+1\right) N_{0}}{2 E\left(N_0\right) N_{0} - E\left(N_0-1\right) N_{0} + E\left(N_0-1\right) - E\left(N_0+1\right) N_{0} - E\left(N_0+1\right)} \\ a_1 &= \frac{- E\left(N_0\right) E\left(N_0-1\right) - E\left(N_0\right) E\left(N_0+1\right) + 2 E\left(N_0-1\right) E\left(N_0+1\right)}{2 E\left(N_0\right) N_{0} - E\left(N_0-1\right) N_{0} + E\left(N_0-1\right) - E\left(N_0+1\right) N_{0} - E\left(N_0+1\right)} \\ b_1 &= \frac{- 2 E\left(N_0\right) + E\left(N_0-1\right) + E\left(N_0+1\right)}{2 E\left(N_0\right) N_{0} - E\left(N_0-1\right) N_{0} + E\left(N_0-1\right) - E\left(N_0+1\right) N_{0} - E\left(N_0+1\right)}\end{split}$

Due to the complexity of the obtained parameters, we skip substituting them into the energy expression. However, at this stage, the energy expression can be evaluated for any given number of electrons as implemented in chemtools.tool.globaltool.RationalGlobalTool.energy.

The derivatives of the energy model with respect to the number of electrons at fixed external potential are:

$\begin{split}\left( \frac{\partial E}{\partial N} \right)_{v(\mathbf{r})} &= \frac{a_1 - a_0 b_1}{\left(1 + b_1 N\right)^2} \\ \left( \frac{\partial^2 E}{\partial N^2} \right)_{v(\mathbf{r})} &= \frac{-2 b_1 \left(a_1 - a_0 b_1\right)}{\left(1 + b_1 N\right)^3} \\ \left( \frac{\partial^n E}{\partial N^n} \right)_{v(\mathbf{r})} &= \frac{(-b_1)^{n - 1} \left(a_1 - a_0 b_1\right) n!}{\left(1 + b_1 N\right)^{n+1}}\end{split}$

These derivatives can be evaluated for any number of electrons as implemented in chemtools.tool.globaltool.RationalGlobalTool.energy_derivative.

In the 3-point rational model, evaluating the first-, second-, and higher-order derivatives of energy evaluated at $$N_0$$ gives the following expressions for the chemical potential, chemical hardness, and hyper-hardnesses,

$\begin{split}\mu = \left. \left( \frac{\partial E}{\partial N} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= \frac{a_1 - a_0 b_1}{\left(1 + b_1 N_0\right)^2} \\ \eta = \left. \left( \frac{\partial^2 E}{\partial N^2} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= \frac{-2 b_1 \left(a_1 - a_0 b_1\right)}{\left(1 + b_1 N_0\right)^3} \\ \eta^{(2)} = \left. \left( \frac{\partial^3 E}{\partial N^3} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= \frac{6 b_1^2 \left(a_1 - a_0 b_1\right)}{\left(1 + b_1 N_0\right)^4} \\ \eta^{(n)} = \left. \left( \frac{\partial^{n+1} E}{\partial N^{n+1}} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= \frac{(-b_1)^n \left(a_1 - a_0 b_1\right) \left(n+1\right)!}{\left(1 + b_1 N_0\right)^{n+2}} \text{ for } n\geq2\end{split}$

These are implemented in chemtools.tool.globaltool.RationalGlobalTool.chemical_potential and chemtools.tool.globaltool.RationalGlobalTool.chemical_hardness.

Using these expressions, one can derive the following expressions for the chemical softness and the low-order hyper-softnesses,

$\begin{split}S = - \left. \left( \frac{\partial^2\Omega}{\partial\mu^2} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= \frac{1}{\eta} = \frac{-\left(1 + b_1 N_0\right)^3}{2 b_1 \left(a_1 - a_0 b_1\right)} \\ S^{(2)} = - \left. \left( \frac{\partial^{3}\Omega}{\partial\mu^{3}} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= -\eta^{(2)} \cdot S^3 \\ &= -\frac{6 b_1^2 \left(a_1 - a_0 b_1\right)}{\left(1 + b_1 N_0\right)^4} \frac{\left(1 + b_1 N_0\right)^9}{2^3 b_1^3 \left(a_1 - a_0 b_1\right)^3} = \frac{3 \left(1 + b_1 N_0\right)^5}{4 b_1 \left(a_1 - a_0 b_1\right)^2} \\ S^{(3)} = - \left. \left( \frac{\partial^{4}\Omega}{\partial\mu^{4}} \right)_{v(\mathbf{r})} \right|_{N = N_0} &= -\eta^{(3)} \cdot S^4 + 3 \left(\eta^{(2)}\right)^2 \cdot S^5 \\ &= -\frac{24 b_1^3 \left(a_1 - a_0 b_1\right)}{\left(1 + b_1 N_0\right)^5} \frac{\left(1 + b_1 N_0\right)^12}{2^4 b_1^4 \left(a_1 - a_0 b_1\right)^4} \\ & + 3\frac{6^2 b_1^4 \left(a_1 - a_0 b_1\right)^2}{\left(1 + b_1 N_0\right)^8} \frac{\left(1 + b_1 N_0\right)^15}{2^5 b_1^5 \left(a_1 - a_0 b_1\right)^5} \\ &= \frac{-15 \left(1 + b_1 N_0\right)^7}{8 b_1 \left(a_1 - a_0 b_1\right)^3}\end{split}$

ChemTools can also compute higher-order hyper-softnesses, using the (extended) inverse function theorem for derivatives. Please refer to Deriving Global Hyper-Softness for details.

To obtain the derived global reactivity indicators for the rational energy model, the maximum number of electrons accepted by the system should be calculated.

Todo

1. Include $$N_{\text{max}}=\infty$$ and derived global reactivity tools

References:

Todo