# Multipole fit with Lagrangian multipliers

Consider the electrostatic potential due to multipoles places at the position of atoms:

$E_{i}=\sum_{j}^{atom}\sum_{n}^{multipole}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}m_{j}^{(n)}$

If the quantum mechanical electrostatic potential is to be minimized, a cost function can written of the form:

$z=\sum_{i}^{point}\left(V_{i,\mathrm{QM}}-E_{i}\right)^{2}+\sum_{l}^{constraints}\lambda_{l}g_{l}$

Expanding the square:

$z=\sum_{i}^{point}\left(V_{i,\mathrm{QM}}^{2}+E_{i}^{2}-2E_{i}V_{i,\mathrm{QM}}\right)+\sum_{l}^{constraints}\lambda_{l}g_{l}$

It is known that in the minima that the derivative with respect to all parameters equals to zero.

$\frac{\partial E_{i}^{2}}{\partial m_{p}^{(k)}}=2\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\sum_{n}^{multipole}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}m_{j}^{(n)}$

It can be written that:

$\sum_{k}^{multipole}\frac{\partial z}{\partial m_{p}^{(k)}}=\sum_{k}^{multipole}\sum_{i}^{point}\left(\sum_{j}^{atom}2\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\sum_{n}^{multipole}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}m_{j}^{(n)}-2V_{i,\mathrm{QM}}\sum_{j}^{atom}\frac{\left(-1\right)^{k}}{k!}T_{ij}^{(k)}\right)$

Since the condition of minima was:

$\sum_{k}^{multipole}\frac{\partial z}{\partial m^{(k)}}=0$

And reducing out the factor 2, it can be written as:

$\sum_{p}^{atom}\sum_{k}^{multipole}\sum_{j}^{atom}\sum_{n}^{multipole}\sum_{i}^{point}\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}m_{j}^{(n)}=\sum_{j}^{atom}\sum_{k}^{multipole}\sum_{i}^{point}V_{i,\mathrm{QM}}\frac{\left(-1\right)^{k}}{k!}T_{ij}^{(k)}$

Now this can be identified as a matrix equation of the following form:

$\overline{\overline{A}}\overline{x}=\overline{B}$

With the elements given as:

$A_{pk,jn}=\sum_{i}^{point}\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}$

and,

$B_{jn}=\sum_{i}^{point}V_{i,\mathrm{QM}}\frac{\left(-1\right)^{k}}{k!}T_{ij}^{(k)}$

and,

$x_{jn}=m_{j}^{(n)}$

## Constraints

For the technique of Lagrangian multiplies, the multipoles $$m_{j}^{(n)}$$ can be constrained. The general constraint for multipoles is given as:

$g_{n}=\sum_{i}^{n}\sum_{j}^{atoms}m_{j}^{(i)}R^{(n-i)}-m_{tot}^{(n)}$

Here $$R^{(m)}$$ is the outer product of the distance between the origin and the multipole $$m$$ times. And $$R^{(0)}=1$$. Thus:

$\frac{\partial z}{\partial\lambda_{n}}=g_{n}=0$

Giving:

$m_{tot}^{(n)}=\sum_{i}^{n}\sum_{j}^{atoms}m_{j}^{(i)}R^{(n-i)}$

Which is the constraints that should be fulfilled.

## Example Matrix

A total matrix equation as (for up to dipoles), with $$d_{i}=r_{i}-R_{\mathrm{cm}}$$:

$\left[\begin{array}{cccccccccccccccc} A_{1,1}^{q,q} & \ldots & A_{1,j}^{q,q} & A_{1,1}^{q,\mu_{x}} & \ldots & A_{1,j}^{q,\mu_{x}} & A_{1,1}^{q,\mu_{y}} & \ldots & A_{1,j}^{q,\mu_{y}} & A_{1,1}^{q,\mu_{z}} & \ldots & A_{1,j}^{q,\mu_{z}} & 1 & d_{1,x} & d_{1,y} & d_{1,z}\\ \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \vdots & \vdots & \vdots\\ A_{p,1}^{q,q} & \ldots & A_{p,j}^{q,q} & A_{p,1}^{q,\mu_{x}} & \ldots & A_{p,j}^{q,\mu_{x}} & A_{p,1}^{q,\mu_{y}} & \ldots & A_{p,j}^{q,\mu_{y}} & A_{p,1}^{q,\mu_{z}} & \ldots & A_{p,j}^{q,\mu_{z}} & 1 & d_{p,x} & d_{p,y} & d_{p,z}\\ A_{1,1}^{\mu_{x},q} & \ldots & A_{1,j}^{\mu_{x},q} & A_{1,1}^{\mu_{x},\mu_{x}} & \ldots & A_{1,j}^{\mu_{x},\mu_{x}} & A_{1,1}^{\mu_{x},\mu_{y}} & \ldots & A_{1,j}^{\mu_{x},\mu_{y}} & A_{1,1}^{\mu_{x},\mu_{z}} & \ldots & A_{1,j}^{\mu_{x},\mu_{z}} & 0 & 1 & 0 & 0\\ \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \vdots & \vdots & \vdots\\ A_{p,1}^{\mu_{x},q} & \ldots & A_{p,j}^{\mu_{x},q} & A_{p,1}^{\mu_{x},\mu_{x}} & \ldots & A_{p,j}^{\mu_{x},\mu_{x}} & A_{p,1}^{\mu_{x},\mu_{y}} & \ldots & A_{p,j}^{\mu_{x},\mu_{y}} & A_{p,1}^{\mu_{x},\mu_{z}} & \ldots & A_{p,j}^{\mu_{x},\mu_{z}} & 0 & 1 & 0 & 0\\ A_{1,1}^{\mu_{y},q} & \ldots & A_{1,j}^{\mu_{y},q} & A_{1,1}^{\mu_{y},\mu_{x}} & \ldots & A_{1,j}^{\mu_{y},\mu_{x}} & A_{1,1}^{\mu_{y},\mu_{y}} & \ldots & A_{1,j}^{\mu_{y},\mu_{y}} & A_{1,1}^{\mu_{y},\mu_{z}} & \ldots & A_{1,j}^{\mu_{y},\mu_{z}} & 0 & 0 & 1 & 0\\ \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \vdots & \vdots & \vdots\\ A_{p,1}^{\mu_{y},q} & \ldots & A_{p,j}^{\mu_{y},q} & A_{p,1}^{\mu_{y},\mu_{x}} & \ldots & A_{p,j}^{\mu_{y},\mu_{x}} & A_{p,1}^{\mu_{y},\mu_{y}} & \ldots & A_{p,j}^{\mu_{y},\mu_{y}} & A_{p,1}^{\mu_{y},\mu_{z}} & \ldots & A_{p,j}^{\mu_{y},\mu_{z}} & 0 & 0 & 1 & 0\\ A_{1,1}^{\mu_{z},q} & \ldots & A_{1,j}^{\mu_{z},q} & A_{1,1}^{\mu_{z},\mu_{x}} & \ldots & A_{1,j}^{\mu_{z},\mu_{x}} & A_{1,1}^{\mu_{z},\mu_{y}} & \ldots & A_{1,j}^{\mu_{z},\mu_{y}} & A_{1,1}^{\mu_{z},\mu_{z}} & \ldots & A_{1,j}^{\mu_{z},\mu_{z}} & 0 & 0 & 0 & 1\\ \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots & \vdots & \vdots & \vdots\\ A_{p,1}^{\mu_{z},q} & \ldots & A_{p,j}^{\mu_{z},q} & A_{p,1}^{\mu_{z},\mu_{x}} & \ldots & A_{p,j}^{\mu_{z},\mu_{x}} & A_{p,1}^{\mu_{z},\mu_{y}} & \ldots & A_{p,j}^{\mu_{z},\mu_{y}} & A_{p,1}^{\mu_{z},\mu_{z}} & \ldots & A_{p,j}^{\mu_{z},\mu_{z}} & 0 & 0 & 0 & 1\\ 1 & \ldots & 1 & 0 & \ldots & 0 & 0 & \ldots & 0 & 0 & \ldots & 0 & 0 & 0 & 0 & 0\\ d_{1,x} & \ldots & d_{j,x} & 1 & \ldots & 1 & 0 & \ldots & 0 & 0 & \ldots & 0 & 0 & 0 & 0 & 0\\ d_{1,y} & \ldots & d_{j,y} & 0 & \ldots & 0 & 1 & \ldots & 1 & 0 & \ldots & 0 & 0 & 0 & 0 & 0\\ d_{1,z} & \ldots & d_{j,z} & 0 & \ldots & 0 & 0 & \ldots & 0 & 1 & \ldots & 1 & 0 & 0 & 0 & 0 \end{array}\right]\left[\begin{array}{c} q_{1}\\ \vdots\\ q_{p}\\ \mu_{x,1}\\ \vdots\\ \mu_{x,p}\\ \mu_{y,1}\\ \vdots\\ \mu_{y,p}\\ \mu_{z,1}\\ \vdots\\ \mu_{z,p}\\ \lambda_{q}\\ \lambda_{\mu_{x}}\\ \lambda_{\mu_{y}}\\ \lambda_{\mu_{z}} \end{array}\right]=\left[\begin{array}{c} B_{1}^{q}\\ \vdots\\ B_{p}^{q}\\ B_{1}^{\mu_{x}}\\ \vdots\\ B_{p}^{\mu_{x}}\\ B_{1}^{\mu_{y}}\\ \vdots\\ B_{p}^{\mu_{y}}\\ B_{1}^{\mu_{z}}\\ \vdots\\ B_{p}^{\mu_{z}}\\ q_{tot}\\ \mu_{x,tot}\\ \mu_{y,tot}\\ \mu_{z,tot} \end{array}\right]$

Or in more compressed form:

$\left[\begin{array}{cccc} A^{q,q} & A^{q,\mu} & 1 & \left(r_{j}-R_{\mathrm{cm}}\right)\\ A^{\mu,q} & A^{\mu,\mu} & 0 & 1\\ 1 & 0 & 0 & 0\\ \left(r_{j}-R_{\mathrm{cm}}\right) & 1 & 0 & 0 \end{array}\right]\left[\begin{array}{c} q\\ \mu\\ \lambda_{q}\\ \lambda_{mu} \end{array}\right]=\left[\begin{array}{c} B^{q}\\ B^{\mu}\\ q_{tot}\\ \mu_{tot} \end{array}\right]$

For the implementation it can be noted that $$\overline{\overline{A}}$$ is symmetric. Now the matrix $$A$$ can be constructed by constructing an auxiliary matrix:

$A_{aux,pk,i}=\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}$

Thus:

$A=A_{aux}\cdot A_{aux}^{T}$

$A_{pk,jn}=\sum_{i}^{point}\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\frac{\left(-1\right)^{n}}{n!}T_{ij}^{(n)}\rightarrow\sum_{i}^{point}\frac{\left(-1\right)^{k}}{\left(2k-1\right)!!}T_{ip}^{(k)}\frac{\left(-1\right)^{n}}{\left(2n-1\right)!!}T_{ij}^{(n)}$
$B_{jn}=\sum_{i}^{point}V_{i,\mathrm{QM}}\frac{\left(-1\right)^{k}}{k!}T_{ij}^{(k)}\rightarrow\sum_{i}^{point}V_{i,\mathrm{QM}}\frac{\left(-1\right)^{k}}{\left(2k-1\right)!!}T_{ip}^{(k)}$
$A_{aux,pk,i}=\frac{\left(-1\right)^{k}}{k!}T_{ip}^{(k)}\rightarrow\frac{\left(-1\right)^{k}}{\left(2k-1\right)!!}T_{ip}^{(k)}$ If you enjoyed this post you can donate a coffee , if you like :)