Jacobi coordinates

In the theory of many-particle systems, Jacobi coordinates often are used to simplify the mathematical formulation. These coordinates are particularly common in treating polyatomic molecules and chemical reactions,[3] and in celestial mechanics.[4] An algorithm for generating the Jacobi coordinates for N bodies may be based upon binary trees.[5] In words, the algorithm is described as follows:[5]

Let m_j and m_k be the masses of two bodies that are replaced by a new body of virtual mass M = m_j + m_k. The position coordinates x_j and x_k are replaced by their relative position r_jk = x_j − x_k and by the vector to their center of mass R_jk = (m_j q_j + m_kq_k)/(m_j + m_k). The node in the binary tree corresponding to the virtual body has m_j as its right child and m_k as its left child. The order of children indicates the relative coordinate points from x_k to x_j. Repeat the above step for N − 1 bodies, that is, the N − 2 original bodies plus the new virtual body.

Jacobi coordinates for two-body problem; Jacobi coordinates are ${\displaystyle {\boldsymbol {R}}={\frac {m_{1}}{M}}{\boldsymbol {x}}_{1}+{\frac {m_{2}}{M}}{\boldsymbol {x}}_{2}}$ and ${\displaystyle {\boldsymbol {r}}={\boldsymbol {x}}_{1}-{\boldsymbol {x}}_{2}}$ with ${\displaystyle M=m_{1}+m_{2}}$.[1]

A possible set of Jacobi coordinates for four-body problem; the Jacobi coordinates are r₁, r₂, r₃ and the center of mass R. See Cornille.[2]

For the N-body problem the result is:[2]

${\displaystyle {\boldsymbol {r}}_{j}={\frac {1}{m_{0j}}}\sum _{k=1}^{j}m_{k}{\boldsymbol {x}}_{k}\ -\ {\boldsymbol {x}}_{j+1}\ ,\quad j\in \{1,2,\dots ,N-1\}}$

${\displaystyle {\boldsymbol {r}}_{N}={\frac {1}{m_{0N}}}\sum _{k=1}^{N}m_{k}{\boldsymbol {x}}_{k}\ ,}$

with

${\displaystyle m_{0j}=\sum _{k=1}^{j}\ m_{k}\ .}$

The vector ${\displaystyle {\boldsymbol {r}}_{N}}$ is the center of mass of all the bodies:

The result one is left with is thus a system of N-1 translationally invariant coordinates ${\displaystyle {\boldsymbol {r}}_{1},\dots ,{\boldsymbol {r}}_{N-1}}$ and a center of mass coordinate ${\displaystyle {\boldsymbol {r}}_{N}}$, from iteratively reducing two-body systems within the many-body system.

This change of coordinates has associated Jacobian equal to ${\displaystyle 1}$.

If one is interested in evaluating a free energy operator in these coordinates, one obtains

${\displaystyle H_{0}=-\sum _{j=1}^{N}{\frac {1}{2m_{j}}}\,\partial _{{\boldsymbol {x}}_{j}}^{2}=-{\frac {1}{2m_{0N}}}\,\partial _{{\boldsymbol {r}}_{N}}^{2}\!-{\frac {1}{2}}\sum _{j=1}^{N-1}\!\left({\frac {1}{m_{j+1}}}+{\frac {1}{m_{0j}}}\right)\partial _{{\boldsymbol {r}}_{j}}^{2}}$

In the calculations can be useful the following identity

${\displaystyle \sum _{k=j+1}^{N}{\frac {m_{k}}{m_{0k}m_{0k-1}}}={\frac {1}{m_{0j}}}-{\frac {1}{m_{0N}}}}$.