# Active space chosen for VQE in qiskit

I encountered a problem when learning from the qiskit tutorial about solving electronic structure problem with VQE. Under the "Running VQE on a Statevector Simulator" part, the code provided for leveraging active space of LiH to reduce the qubit requirement seems quite confusing. The freeze_list in the code is interpreted as the core space in my opinion. However, what confuse me is that why to choose the [-3, -2] orbitals as virtual space to be removed (remove_list). Or the problem can be stated as why to choose [-3, -2] as the virtual space of the LiH molecule? The corresponding code of the tutorial is provide as below:

def get_qubit_op(dist):
driver = PySCFDriver(atom="Li .0 .0 .0; H .0 .0 " + str(dist), unit=UnitsType.ANGSTROM,
charge=0, spin=0, basis='sto3g')
molecule = driver.run()
freeze_list = [0]
remove_list = [-3, -2]
repulsion_energy = molecule.nuclear_repulsion_energy
num_particles = molecule.num_alpha + molecule.num_beta
num_spin_orbitals = molecule.num_orbitals * 2
remove_list = [x % molecule.num_orbitals for x in remove_list]
freeze_list = [x % molecule.num_orbitals for x in freeze_list]
remove_list = [x - len(freeze_list) for x in remove_list]
remove_list += [x + molecule.num_orbitals - len(freeze_list)  for x in remove_list]
freeze_list += [x + molecule.num_orbitals for x in freeze_list]
ferOp = FermionicOperator(h1=molecule.one_body_integrals, h2=molecule.two_body_integrals)
ferOp, energy_shift = ferOp.fermion_mode_freezing(freeze_list)
num_spin_orbitals -= len(freeze_list)
num_particles -= len(freeze_list)
ferOp = ferOp.fermion_mode_elimination(remove_list)
num_spin_orbitals -= len(remove_list)
qubitOp = ferOp.mapping(map_type='parity', threshold=0.00000001)
qubitOp = Z2Symmetries.two_qubit_reduction(qubitOp, num_particles)
shift = energy_shift + repulsion_energy
return qubitOp, num_particles, num_spin_orbitals, shift