One-dimensional (1D) confined pnictogen shows a diverse range of allotropes and potential applications in electronic devices and the chemical industry. Here, we report a theoretical study aimed at an accurate assessment of the thermodynamic stability of pnictogen structures under nano-meter confinements. We develop a cylindrical potential for pnictogen, which can be integrated with density functional theory to model a confined system towards achieving ab initio accuracy. We discuss in detail the performance of confining potentials and provide insights into the understanding of complex interactions between confined pnictogen and carbon nanotubes. We reassess the thermodynamic stability of 1D pnictogens in carbon nanotubes, explaining the diverse features of confined pnictogens in recent experimental and theoretical studies.