Add chelpg charges in qmmm folder. (#1)

* Add chelpg charges in qmmm folder.

* Update chelpg.py

* Update chelpg.py

* Add unit test for chelpg, and compare with Qchem

* Add an example to calculate chelpg

examples/15-chelpg.py

@@ -0,0 +1,39 @@
+# Copyright 2023 The GPU4PySCF Authors. All Rights Reserved.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from pyscf import gto
+from gpu4pyscf.dft import rks
+from gpu4pyscf.qmmm import chelpg
+
+
+mol = gto.Mole()
+mol.verbose = 0
+mol.output = None
+mol.atom = [
+    [1 , (1. ,  0.     , 0.000)],
+    [1 , (0. ,  1.     , 0.000)],
+    [1 , (0. , -1.517  , 1.177)],
+    [1 , (0. ,  1.517  , 1.177)] ]
+mol.basis = '631g'
+mol.unit = 'B'
+mol.build()
+mol.verbose = 6
+
+xc = 'b3lyp'
+mf = rks.RKS(mol, xc=xc)
+mf.grids.level = 5
+mf.kernel()
+q = chelpg.eval_chelpg_layer_gpu(mf)
+print(q) # [ 0.04402311  0.11333945 -0.25767919  0.10031663]

gpu4pyscf/qmmm/__init__.py

@@ -0,0 +1,16 @@
+# Copyright 2023 The GPU4PySCF Authors. All Rights Reserved.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from gpu4pyscf.qmmm import chelpg