  We exploit a prescription to observe directly the physical properties of the
thermodynamic limit under continuously applied field in one-dimensional quantum
finite lattice systems. By systematically scaling down the energy of the
Hamiltonian of the open system from center toward both ends, one could adopt
the edge sites with negligibly small energy scale as a "grand canonical" small
particle bath, and an equilibrium states with non-integer arbitrary conserved
numbers, e.g., electron numbers or sz, are realized in the main part of the
system. This will enable the evaluation of the response functions under
continuously varying external field in a small lattice without any fine tuning
or scaling of parameters while keeping the standard numerical accuracy.
Demonstrations are given on quantum spin systems as well as on a Hubbard model
by the density matrix renormalization group calculation.
