The finite-size scaling method is used to calculate the critical parameters for the lithium isoelectronic series. The critical nuclear charge, which is the minimum charge necessary to bind three electrons, for the ground state was found to be Z c Ӎ 2. Results show that the analytical behavior of the energy as a function of the nuclear charge for lithium is completely different from that of helium. Analogy with standard phase transitions show that for helium, the transition from a bound state to a continuum is "first order," while lithium exhibits a "second order phase transition." [S0031-9007(98)06405-9] PACS numbers: 05.70.Jk The study of the critical parameters of a quantum Hamiltonian and quantum phase transitions have attracted much interest in recent years [1]. These transitions take place at the absolute zero of temperature, where phase transition means that the quantum ground state of the system changes in some fundamental way as some microscopic parameters change in the Hamiltonian. In atomic and molecular physics, it has been suggested that there are possible analogies between critical phenomena and singularities of the energy [2][3][4]. Analogies between symmetry breaking of electronic structure configurations and standard phase transitions have been established for many electron atoms and simple molecular systems by studying the corresponding large dimension limit Hamiltonian [5,6].Recently, we presented the finite-size scaling (FSS) method to calculate the critical parameters for electronic structure [7,8]. In statistical mechanics, the FSS method provides a way to extrapolate information obtained from a finite system to the thermodynamic limit [9,10]. In our applications, the finite size corresponds to the number of elements in a complete basis set used to expand the exact eigenfunction of a given Hamiltonian. In the FSS method we assumed that the two lowest eigenvalues of the quantum Hamiltonian could be taken as the leading eigenvalues of a transfer matrix of a classical pseudosystem. The phenomenological renormalization equation was used to obtain the critical properties of the system. In this approach one has to rely on the analogy to classical statistical mechanics, a more direct finite-size scaling approach without the need to make such an analogy was established by a systematic expansion in a finite (truncated) basis set [11].The analytical behavior of the energy as a function of parameters for a given system has been the subject of study for many years. In particular, the study of the analytical behavior of the energy as a function of the nuclear charge, Z. Morgan and co-workers [12] have performed a 401-order perturbation calculation to resolve the controversy over the radius of convergence of the l 1͞Z expansion for the ground state energy of the heliumlike ions. Such high order calculations were necessary to study the asymptotic behavior of the perturbation series and to determine that the radius of convergence, l ء , is equal to l c , the critical value of l for which the Hamiltonia...