class_docstr: BaseDF represents the density fitting object used in calculating the
  electron repulsion (and xc energy) in Hamiltonian.
  
  Density fitting in density functional theory (DFT) is a technique used to
  reduce the computational cost of evaluating electron repulsion integrals.
  In DFT, the key quantity is the electron density rather than the wave
  function, and the electron repulsion integrals involve four-electron
  interactions, making them computationally demanding.
  
  Density fitting exploits the fact that many-electron integrals can be
  expressed as a sum of two-electron integrals involving auxiliary basis
  functions. By approximating these auxiliary basis functions, often referred
  to as fitting functions, the computational cost can be significantly reduced.
  
  Examples
  --------
  >>> from deepchem.utils.dft_utils import BaseDF
  >>> import torch
  >>> class MyDF(BaseDF):
  ...     def __init__(self):
  ...         super(MyDF, self).__init__()
  ...     def get_j2c(self):
  ...         return torch.ones((3, 3))
  ...     def get_j3c(self):
  ...         return torch.ones((3, 3, 3))
  >>> df = MyDF()
  >>> df.get_j2c()
  tensor([[1., 1., 1.],
          [1., 1., 1.],
          [1., 1., 1.]])
docstr: Construct the matrices required to perform the calculation and return
  self.
  
  Returns
  -------
  BaseDF
      The constructed density fitting object.
code: def build(self) -> BaseDF:
      """
      Construct the matrices required to perform the calculation and return
      self.
  
      Returns
      -------
      BaseDF
          The constructed density fitting object.
  
      """
      pass
  
file_path: deepchem/deepchem/utils/dft_utils/df/base_df.py
imports: from __future__ import annotations
  from abc import abstractmethod, abstractproperty
  from typing import List
  import torch
  from deepchem.utils.differentiation_utils import EditableModule, LinearOperator
label_desc: Use image node_id 1 for calling the BaseDF obj's underlying member method code with example usage: obj.build() without return types