{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "from itertools import chain\n",
    "import numpy as np\n",
    "import math\n",
    "from scipy import sparse\n",
    "from scipy.sparse.linalg import eigsh\n",
    "\n",
    "#sample potential\n",
    "def V_gauss(dr_sqr):\n",
    "    return -4*math.exp(-dr_sqr/4)\n",
    "\n",
    "n=3 # no of particles\n",
    "L=14\n",
    "N=6 # no of lattice points\n",
    "mu=1/2 # reduced mass\n",
    "\n",
    "DOF=(n-1)*3 # degrees of freedom after excluding CM\n",
    "\n",
    "s=np.arange(N**DOF) # matrix index\n",
    "\n",
    "# k index for each particle and each dimension\n",
    "k=np.empty((n-1,3),dtype=np.dtype)\n",
    "for dof in range(DOF):\n",
    "    k[dof//3,dof%3]=(s%N**(DOF-dof))//N**(DOF-1-dof)-N//2\n",
    "\n",
    "x=k*(L/N) # x coordinate from k index\n",
    "\n",
    "# adding up all non-local interactions\n",
    "V_local=np.zeros(N**DOF)\n",
    "\n",
    "# 2-body interactions\n",
    "V2=np.vectorize(V_gauss)\n",
    "dxs=chain((x[i,:] for i in range(n-1)), (x[i,:]-x[j,:] for (i,j) in np.ndindex((n-1,n-1)) if i<j)) # with last particle + with each other\n",
    "for dx in dxs:\n",
    "    dr_sqr=np.sum(dx*dx)\n",
    "    V_local+=V2(dr_sqr)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "filename = \"temp/V_vals.dat\"\n",
    "\n",
    "with open(filename, 'br') as f:\n",
    "    buffer = f.read()\n",
    "\n",
    "V_julia = np.frombuffer(buffer, dtype=np.float32)\n",
    "\n",
    "abs_diff = np.abs(V_local-V_julia)\n",
    "s = np.mean(abs_diff)\n",
    "print(s)"
   ]
  }
 ],
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