code: private Matrix calculateV(IBasis basis) {
          int size = basis.getSize();
          Matrix V = new Matrix(size, size);
          int i, j;
          for (i = 0; i < size; i++)
              for (j = 0; j < size; j++)
                  V.matrix[i][j] = basis.calcV(i, j);
  
          return V;
      }
docs: Calculates the matrix for the potential matrix
  
  V_i,j = <chi_i | 1/r | chi_j>