Example added for PBC-PDFT

examples/pbc/pdft_gamma_point.py

@@ -0,0 +1,74 @@
+
+'''
+Here, I am doing the gamma-point CASSCF followed by PDFT.
+Note: 
+    1. mf.exxdiv=None should be used. Post-SCF method require this.
+    2. I am using GDF. In case of default DF which is FFTDF, the grids 
+    for the periodic DFT will change. Which I haven't tested yet.
+    3. Also, the MCPDFT (tPBE) == PBE with SD active space and same mo_coeff.
+'''
+
+
+POSCAR='''
+Polyacetylene Unit Cell
+1.0
+2.4700000000  0.0000000000  0.0000000000
+0.0000000000  17.5000000000  0.0000000000
+0.0000000000  0.0000000000  17.5000000000
+C    H
+2   2
+Cartesian
+-0.5892731038  0.3262391909  0.0000000000
+0.5916281105  -0.3261693897  0.0000000000
+-0.5866101958  1.4126530287  0.0000000000
+0.5889652025  -1.4125832275  0.0000000000
+'''
+
+import numpy as np
+from pyscf.pbc import gto, scf, dft
+from pyscf import mcscf
+from mrh.my_pyscf import mcpdft
+
+# Periodic Calculation for CH=CH uni, using CASCI vs RHF
+def getcell():
+    cell = gto.Cell()
+    cell.a='''
+    2.4700000000  0.0000000000  0.0000000000
+    0.0000000000  17.5000000000  0.0000000000
+    0.0000000000  0.0000000000  17.5000000000
+    '''
+    cell.atom='''
+    C -0.5892731038  0.3262391909  0.0000000000
+    C 0.5916281105  -0.3261693897  0.0000000000
+    H -0.5866101958  1.4126530287  0.0000000000
+    H 0.5889652025  -1.4125832275  0.0000000000
+    '''
+    cell.basis = '321g'
+    cell.precision=1e-12
+    cell.verbose = 4
+    cell.build()
+    return cell
+
+def periodicDFT():
+    cell = getcell()
+    mf = scf.RKS(cell).density_fit()
+    mf.verbose=4
+    mf.exxdiv=None
+    mf.xc='pbe'
+    eperpbe = mf.kernel()
+    mc = mcpdft.CASCI(mf, 'tPBE', 1,2)
+    epdftper = mc.kernel(mf.mo_coeff)[0]
+    print("Periodic Cal (PBE vs tPBE): ", np.allclose(eperpbe,epdftper, 1e-7))
+
+def periodicHF():
+    cell = getcell()
+    mf = scf.RHF(cell).density_fit()
+    mf.exxdiv=None
+    eper = mf.kernel()
+    mc = mcscf.CASCI(mf, 1,2)
+    ecasper = mc.kernel(mf.mo_coeff)[0]
+    print("Periodic Cal (CASCI vs RHF): ", np.allclose(eper,ecasper, 1e-7))
+
+if __name__ == "__main__":
+    periodicHF()
+    periodicDFT()