I'm setting up a simple Hartree-Fock problem in pyscf with ECPs to test things out. However, the numbers I get do not make sense to me as the energies are very different. I have never used ECPs before so I don't know exactly what to expect. Here is my code and outputs. Without ECPs (using sto-3g) I get
mol = gto.M(atom="Li 0. 0. 0.; H 0. 0. 1.", basis={'Li':'sto3g', 'H':'sto3g'}, verbose=0)
mf = scf.RHF(mol)
print(mf.kernel())
Output: -7.76736
and with ECP for lithium I get
mol = gto.M(atom="Li 0. 0. 0.; H 0. 0. 1.", basis={'Li':'stuttgart', 'H':'sto3g'}, ecp = {'Li':'stuttgart'}, verbose=0)
mf = scf.RHF(mol)
print(mf.kernel())
Output: -0.63098
As you can notice the energies are very different in these two cases. Is this normal or is something wrong here?