The article by Amy Y. Liu and Marvin L. Cohen explores the prediction of new low compressibility solids, focusing on covalent solids formed between carbon and nitrogen. An empirical model and ab initio calculations are used to suggest that hypothetical covalent solids between carbon and nitrogen could have bulk moduli comparable to or greater than diamond, making them potential candidates for extremely hard materials. The authors choose a prototype system, $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$, and perform first principles pseudopotential total energy calculations. The results are consistent with the empirical model, indicating that materials like $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$ can have bulk moduli similar to diamond. The study also discusses the structural and electronic properties of $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$, including its bond length, ionicity, and valence charge density. The authors conclude that it may be possible to synthesize such materials in the laboratory, potentially leading to new high-performance materials with applications in engineering and scientific research.The article by Amy Y. Liu and Marvin L. Cohen explores the prediction of new low compressibility solids, focusing on covalent solids formed between carbon and nitrogen. An empirical model and ab initio calculations are used to suggest that hypothetical covalent solids between carbon and nitrogen could have bulk moduli comparable to or greater than diamond, making them potential candidates for extremely hard materials. The authors choose a prototype system, $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$, and perform first principles pseudopotential total energy calculations. The results are consistent with the empirical model, indicating that materials like $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$ can have bulk moduli similar to diamond. The study also discusses the structural and electronic properties of $\beta-\mathrm{C}_{3} \mathrm{~N}_{4}$, including its bond length, ionicity, and valence charge density. The authors conclude that it may be possible to synthesize such materials in the laboratory, potentially leading to new high-performance materials with applications in engineering and scientific research.