2 if structure optimizations are performed within non-spin polarized calculations as has been frequently pointed out in the literature. 39,42,70 122 and 111 systems. The former discrepancy in the 11 compounds was soon resolved by a new DFT calculation based on the full potential linear muffin tin orbital (FPLMTO) method which reconciles the theory of itinerant magnetism with the existing experiments on Fe$_{1+}$$_{x}$Te $^{67}$. It shows that, while the Fermi surface is nested at ( π, π ) in the undoped FeTe as in other iron-based superconductors, doping with 0.5 electrons due to the excess of Fe in Fe$_{1+}$$_{x}$Te leads to a strong ( π, 0) nesting of the Fermi surface which cor- responds to the observed magnetic ordering. However, up to now, the second question of enhanced Fermi sur- face nesting in 1111 versus suppression in 122 and 111 compounds when As is substituted by Sb still remains. We used high energy cutoffs of 408 eV and 1632 eV for the wave functions and charge density expansion, respectively. The total energy was converged to less than 0.01 meV/atom and the cell parameters to less than 0.0005 ˚ A. Part of our results are double-checked by the full potential linearized augmented plane wave (FPLAPW) method as implemented in the WIEN2k code 71 and full potential local orbital (FPLO) method $^{72}$. Results are consistent among these methods. Through- out the paper, the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) to DFT has been used if not specified otherwise, and comparisons with the results from local density approximation (LDA) were also per- formed. In order to determine if the Fermi surface nesting is the driving force for the low-temperature stripe-type antiferromagnetic ordering, we calculate the q -dependent Pauli susceptibility at ω =0 and Fermi surface cuts at different k$_{z}$ planes without magnetization. These cal- culations were performed with the WIEN2k code using RK$_{max}$ = 7. While 40000 k points in each k$_{z}$ plane are used in calculating Fermi surface cuts, a three dimen- sional grid of 128 × 128 × 128 k and q points and the constant matrix element approximation are employed for the susceptibility. For the calculations including Ce, Nd, and Sm atoms in the nonmagnetic phases, we apply the open core approximation for the localized f electrons. All calculations were performed in the scalar relativistic ap- proximation, which usually provides a good description of structural properties even for heavier elements $^{73}$. Thus, spin-orbit coupling, which could be potentially relevant especially for Bi compounds $^{74,75}$, is neglected in the cal- culations for the valence electrons. However, since most of the weight of the Bi 6p states is well below the Fermi energy and irrelevant to magnetic ordering, we would ex- pect only minor modifications for the resulting Fermi sur- faces and susceptibilities. In this work, by applying Car-Parrinello molecular dy- namics 68 based on a projector augmented wave (PAW) basis $^{69}$, we will show that, in contrast to the results from a pseudopotential method $^{65}$, the magnetic moment and the Pauli susceptibility at ( π, π ) are simultaneously en- hanced when As is replaced by Sb in LaOFeAs, BaFe$_{2}$As$_{2}$ and LiFeAs, which strongly suggests that Fermi sur- face nesting is closely related to the magnetic moment strength, and the itinerant scenario of nesting-driven magnetism is still valid in the 111, 122 and 1111 com- pounds. By further comparing the Pauli susceptibilities of LaOFe Pn with Pn =P, As, Sb, and Bi, we argue that the absence and the presence of magnetism at ambient pressure in LaOFeP and LaOFeAs, respectively, origi- nate from the competition between the instabilities of the susceptibility at (0 , 0) and ( π, π ), which again in- dicates the importance of Fermi surface nesting for the description of magnetism. We predict that a stripe- type antiferromagnetic metallic state should be present in the hypothetical compounds LaOFeSb and LaOFeBi. Finally, we study the structural and magnetic proper- ties of 1111 compounds including RE OFeAs ( RE =Ce, Nd, Sm), LaOFe Pn ( Pn =P, As, Sb, Bi) and ScOFe Pn ( Pn =P, As) and predict that LaOFeSb could be a su- perconductor with the highest transition temperature among these compounds. Throughout this paper, the Car-Parrinello 68 projector- augmented wave 69 method is employed to optimize the lattice parameters and internal atomic positions. These optimized structures are then used for all subsequent electronic structure calculations unless stated otherwise. 4 × 4 × 4 k $_{-points and doubled (}$√$_{2}$ × √$_{2}$ × 1) unit cells with stripe-type antiferromagnetic order are used when relaxation of all lattice and electronic degrees of free- dom is performed. We use time steps of 0.12 fs and fric- tion to cool the systems to zero temperature. Note that the structure optimization is performed in the magnetic phase. As we shall show below, whenever experimental structures are available, our optimized structures com- pare well with the experimental ones. This is not the case As is pointed out in Section I, a DFT calculation based on a pseudopotential method within the SIESTA code 65 reveals a disconnection between magnetism and Fermi surface nesting in 122 and 111 compounds, ( i.e. , while magnetization is enhanced, the Pauli susceptibility at q = ( π, π ) which is responsible for stripe-type antifer- romagnetic ordering is suppressed when As is replaced by Sb in BaFe$_{2}$As$_{2}$ and LiFeAs), and therefore ques- tions the scenario of an itinerant nature of magnetism. From our spin-polarized GGA calculations for LaOFe Pn , BaFe$_{2}$ Pn$_{2}$ and LiFe Pn ( Pn = As, Sb), the same trends in magnetism are detected as observed in Ref. 65: the ground states are all found to be stripe-type antiferro- III. PAULI SUSCEPTIBILITIES AND MAGNETISM IN 1111, 122 AND 111 COMPOUNDS II. METHOD