Dedicated to Professor Rudolf Hoppe on the occasion of his 90th birthdayThe prediction and identification of stable and particularly of metastable compounds is important in achieving innovative materials. [1] With this objective in mind, approaches containing both theoretical examinations of phase stabilities and concepts of a rational synthesis [2] are gaining increasing importance. Thus, an investigation of energy landscapes [3] provides important information about local and global minima to predict the existence of new compounds and structures.Herein, we introduce a combination of quantum-chemical calculations and thermodynamic considerations to realize target-oriented planning and optimization of chemical synthesis. The analysis of phase formation is acquired with an in situ method for monitoring gas-phase reactions. Using the system P-As, we investigated a textbook example of monotropic phase transitions, which features a variety of known and postulated allotropes. [4] To estimate the relative stabilities of the compounds under discussion, structure allotropes of N, P, and As were modeled by means of DFT calculations. The calculated electronic energies (normalized to one atom Pn) for molecular Pn 2 and Pn 4 , the black/orthorhombic (o-Pn), gray/ trigonal (t-Pn), and the simple-cubic (c-Pn) allotrope as well as the tubular polymeric forms of Hittorf (H-Pn), [5] Ruck (R-Pn), [6] and Pfitzner (P-Pn), [7] which are known for phosphorus, are shown in Figure 1. The high stability of N 2 can be observed as well as the preference for solid-state structures for P and As. Figure 1 b emphasizes the results for the t and o forms of P and As. The known stability of o-P compared to the high-pressure modifications t-P and c-P is correctly predicted as well as the stability of gray t-As compared to the known high-pressure modification c-As, hypothetical tubular As allotropes, and predicted o-As. Does this mean that o-As can be synthesized as a metastable compound? [8a] The calculated values of the total electronic energies correctly express the higher stability of o-P compared with the high-pressure phase t-P and also t-As compared to o-As. From the computed values DE el = E el (o)ÀE el (t) of À4 kJ mol À1 for P and + 2.5 kJ mol À1 for As, a transition from the o-to the tstructure in the ideal solution As x P 1Àx can be estimated to be about x = 0.6 ( Figure 2). Taking thermodynamic energy terms into consideration (for details, see the Supporting Information), a distinctive stabilization of the o phase is calculated and the o-t transition shifts to a higher As content (x = 0.9). According to the calculated values, very low energy differences decide whether or not o-As can actually be synthesized. [8b]