_createE_matrix() {
    const nodesArray = this.body.nodeIndices;
    const nodes = this.body.nodes;
    this.E_matrix = {};
    this.E_sums = {};
    for (let mIdx = 0; mIdx < nodesArray.length; mIdx++) {
      this.E_matrix[nodesArray[mIdx]] = [];
    }
    for (let mIdx = 0; mIdx < nodesArray.length; mIdx++) {
      const m = nodesArray[mIdx];
      const x_m = nodes[m].x;
      const y_m = nodes[m].y;
      let dE_dx = 0;
      let dE_dy = 0;
      for (let iIdx = mIdx; iIdx < nodesArray.length; iIdx++) {
        const i = nodesArray[iIdx];
        if (i !== m) {
          const x_i = nodes[i].x;
          const y_i = nodes[i].y;
          const denominator =
            1.0 / Math.sqrt((x_m - x_i) ** 2 + (y_m - y_i) ** 2);
          this.E_matrix[m][iIdx] = [
            this.K_matrix[m][i] *
              (x_m - x_i - this.L_matrix[m][i] * (x_m - x_i) * denominator),
            this.K_matrix[m][i] *
              (y_m - y_i - this.L_matrix[m][i] * (y_m - y_i) * denominator),
          ];
          this.E_matrix[i][mIdx] = this.E_matrix[m][iIdx];
          dE_dx += this.E_matrix[m][iIdx][0];
          dE_dy += this.E_matrix[m][iIdx][1];
        }
      }
      //Store sum
      this.E_sums[m] = [dE_dx, dE_dy];
    }
  }
Create matrix with all energies between nodes
 @private