add the doc for GPU and solvent models (#130)

* Create gpu.rst

* add gpu.rst in user.rst

* add gpu.rst in user.rst

* Update gpu.rst

* Update solvent.rst

* Update solvent.rst

* Update solvent.rst

* added installation

* update solvent models

source/user.rst

@@ -33,4 +33,6 @@ functionalities.
    user/lo.rst
    user/sgx.rst
    user/geomopt.rst
+   user/gpu.rst
    user/reference.rst
+

source/user/gpu.rst

@@ -0,0 +1,154 @@
+.. _user_gpu:
+
+GPU Acceleration (GPU4PySCF)
+****************************
+
+*Modules*: :py:mod:`gpu4pyscf`
+
+.. module:: GPU4PySCF
+   :synopsis: GPU4PySCF
+.. sectionauthor:: Xiaojie Wu <[email protected]>.
+
+Introduction
+============
+
+Modern GPUs accelerate quantum chemistry calculation significantly, but also have an advantage in cost saving `[1]`_.
+Some of basic PySCF modules, such as SCF and DFT, are accelerated with GPU via a plugin package
+GPU4PySCF (See the end of this page for the supported functionalities). For the density fitting scheme,
+GPU4PySCF on A100-80G can be 1000x faster than PySCF on single-core CPU. The speedup of direct SCF scheme is relatively low.
+
+.. _[1]: https://arxiv.org/abs/2404.09452
+
+Installation
+============
+The binary package of GPU4PySCF is released based on the CUDA version.
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+  * - CUDA version
+    - GPU4PySCF
+    - cuTensor
+  * - CUDA 11.x
+    - ``pip3 install gpu4pyscf-cuda11x``
+    - ``pip3 install cutensor-cu11``
+  * - CUDA 12.x
+    - ``pip3 install gpu4pyscf-cuda12x``
+    - ``pip3 install cutensor-cu12``
+
+Usage of GPU4PySCF
+==================
+GPU4PySCF APIs are designed to be compatible with PySCF. When supported, high-level functions and objects are named the same as PySCF. But, GPU4PySCF classes do not directly inherit from PySCF class.
+PySCF objects and GPU4PySCF objects can be converted into each other by :func:`to_gpu` and :func:`to_cpu`. In the conversion, the numpy arrays will be converted into cupy array. And the functions will be omitted if they are not supported with GPU acceleration.
+
+One can use the two modes to accelerate the calculations: directly use GPU4PySCF object::
+
+    import pyscf
+    from gpu4pyscf.dft import rks
+
+    atom ='''
+    O       0.0000000000    -0.0000000000     0.1174000000
+    H      -0.7570000000    -0.0000000000    -0.4696000000
+    H       0.7570000000     0.0000000000    -0.4696000000
+    '''
+
+    mol = pyscf.M(atom=atom, basis='def2-tzvpp')
+    mf = rks.RKS(mol, xc='LDA').density_fit()
+
+    e_dft = mf.kernel()  # compute total energy
+    print(f"total energy = {e_dft}")
+
+    g = mf.nuc_grad_method()
+    g_dft = g.kernel()   # compute analytical gradient
+
+    h = mf.Hessian()
+    h_dft = h.kernel()   # compute analytical Hessian
+
+Alternatively, one can convert PySCF object to the corresponding GPU4PySCF object with :func:`to_gpu` since PySCF 2.5.0 ::
+
+    import pyscf
+    from pyscf.dft import rks
+
+    atom ='''
+    O       0.0000000000    -0.0000000000     0.1174000000
+    H      -0.7570000000    -0.0000000000    -0.4696000000
+    H       0.7570000000     0.0000000000    -0.4696000000
+    '''
+
+    mol = pyscf.M(atom=atom, basis='def2-tzvpp')
+    mf = rks.RKS(mol, xc='LDA').density_fit().to_gpu()  # move PySCF object to GPU4PySCF object
+    e_dft = mf.kernel()  # compute total energy
+
+
+When the GPU task is done, the GPU4PySCF object can be converted into the corresponding PySCF object via :func:`mf.to_cpu()`.
+Then, more sophisticated methods in PySCF can apply. One can also convert the individual CuPy array to numpy array with `Cupy APIs`_.
+
+.. Cupy APIs: https://docs.cupy.dev/en/stable/user_guide/index.html
+
+Functionalities supported by GPU4PySCF
+======================================
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+  * - Method
+    - SCF
+    - Gradient
+    - Hessian
+  * - direct SCF
+    - O
+    - GPU
+    - CPU
+  * - density fitting
+    - O
+    - O
+    - O
+  * - LDA
+    - O
+    - O
+    - O
+  * - GGA
+    - O
+    - O
+    - O
+  * - mGGA
+    - O
+    - O
+    - O
+  * - hybrid
+    - O
+    - O
+    - O
+  * - unrestricted
+    - O
+    - O
+    - O
+  * - PCM solvent
+    - GPU
+    - GPU
+    - FD
+  * - SMD solvent
+    - GPU
+    - GPU
+    - FD
+  * - dispersion correction
+    - CPU*
+    - CPU*
+    - FD
+  * - nonlocal correlation
+    - O
+    - O
+    - NA
+  * - ECP
+    - CPU
+    - CPU
+    - CPU
+  * - MP2
+    - GPU
+    - CPU
+    - CPU
+  * - CCSD
+    - GPU
+    - CPU
+    - NA