added selected CI example for molecules

examples/fci/02-selected_ci_mol.py

@@ -0,0 +1,44 @@
+
+#!/usr/bin/env python
+#
+# Author: Chong Sun <[email protected]>
+#
+'''
+Selected CI for molecules.
+'''
+
+from pyscf import gto, scf, ao2mo, fci
+
+mol = gto.Mole()
+mol.atom = "H 0 0 0; H 0 0 1.2; H 0 0 2.4; H 0 0 3.6"
+mol.unit='angstrom'
+mol.basis = "631g"
+mol.build()
+
+# Run RHF 
+# No need to run UHF because SHCI is very accurate.
+mf = scf.RHF(mol) 
+mf.kernel()
+
+# Rotate the Hamiltonians
+norb = mol.nao
+nelec = mol.nelectron
+mo_coeff = mf.mo_coeff
+h1e = mf.get_hcore()
+eri = mf._eri
+h1e_mo = mo_coeff.T @ h1e @ mo_coeff
+eri_mo = ao2mo.kernel(eri, mo_coeff, compact=False).reshape(norb, norb, norb, norb)
+e_nuc = mf.energy_nuc()
+
+# Run SCI
+scisolver = fci.SCI()
+scisolver.max_cycle = 100
+scisolver.conv_tol = 1e-8
+e, civec = scisolver.kernel(h1e_mo, eri_mo, norb, nelec)
+e_sci = e + e_nuc # add nuclear energy
+print("Selected CI energy: {}".format(e_sci))
+
+# Compared to FCI
+fcisolver = fci.FCI(mf)
+e_fci, fcivec = fcisolver.kernel() 
+print("Difference compared to FCI: {}".format(e_fci - e_sci))